diff --git a/dog-food-calculator-widget.js b/dog-food-calculator-widget.js
index 1051a8d..15f26e9 100644
--- a/dog-food-calculator-widget.js
+++ b/dog-food-calculator-widget.js
@@ -3,9 +3,16 @@
  * Embeddable JavaScript widget for calculating dog's daily calorie requirements
  * by Canine Nutrition and Wellness - https://caninenutritionandwellness.com
  * 
- * Usage: 
+ * Basic Usage: 
  * <script src="https://yourdomain.com/dog-calculator-widget.js"></script>
  * <div id="dog-calorie-calculator"></div>
+ * 
+ * Advanced Usage with theme option:
+ * <script src="https://yourdomain.com/dog-calculator-widget.js"></script>
+ * <div id="dog-calorie-calculator" data-theme="dark"></div>
+ * 
+ * Or with JavaScript:
+ * new DogCalorieCalculatorWidget(container, { theme: 'light' }); // 'light', 'dark', 'system'
  */
 
 (function() {
@@ -399,108 +406,215 @@
             }
         }
 
+        /* Dark theme - manual override */
+        .dog-calc-widget.dog-calc-theme-dark {
+            color: #f5f3f7;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-section,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-collapsible {
+            background: #24202d;
+            border-color: #433c4f;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-collapsible-header {
+            background: #312b3b;
+            border-color: #433c4f;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-collapsible-header:hover {
+            background: #3a3446;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-section h2,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-collapsible-header h3,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group label,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-result-label {
+            color: #f5f3f7;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group select,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group input[type="number"],
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group input[type="text"] {
+            background-color: #312b3b;
+            border-color: #433c4f;
+            color: #f5f3f7;
+            background-image: url("data:image/svg+xml;charset=UTF-8,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%23f5f3f7' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3e%3cpolyline points='6 9 12 15 18 9'%3e%3c/polyline%3e%3c/svg%3e");
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group select option {
+            background-color: #312b3b;
+            color: #f5f3f7;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group select:focus,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group input[type="number"]:focus,
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group input[type="text"]:focus {
+            background-color: #312b3b;
+            border-color: #f19a5f;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-form-group input[readonly] {
+            background-color: #433c4f;
+            color: #b8b0c2;
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-results {
+            background: linear-gradient(135deg, rgba(241, 154, 95, 0.15) 0%, rgba(241, 154, 95, 0.08) 100%);
+            border-color: rgba(241, 154, 95, 0.3);
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-result-value {
+            color: #f5f3f7;
+            background: rgba(241, 154, 95, 0.2);
+        }
+            
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-footer {
+            background: #24202d;
+            border-color: #433c4f;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-unit-label {
+            color: #b8b0c2;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-unit-label.active {
+            color: #f5f3f7;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-slider {
+            background-color: #433c4f;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-action-buttons {
+            background: #312b3b;
+            border-color: #433c4f;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-btn {
+            background: #433c4f;
+            border-color: #433c4f;
+            color: #f5f3f7;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-btn:hover {
+            background: #524a5f;
+            border-color: #524a5f;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-btn-share:hover {
+            border-color: #9f5999;
+            color: #f19a5f;
+        }
+
+        .dog-calc-widget.dog-calc-theme-dark .dog-calc-btn-embed:hover {
+            border-color: #7fa464;
+            color: #7fa464;
+        }
+
+        /* System theme - follows user's OS preference */
         @media (prefers-color-scheme: dark) {
-            .dog-calc-widget {
+            .dog-calc-widget.dog-calc-theme-system {
                 color: #f5f3f7;
             }
             
-            .dog-calc-section,
-            .dog-calc-collapsible {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-section,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-collapsible {
                 background: #24202d;
                 border-color: #433c4f;
             }
             
-            .dog-calc-collapsible-header {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-collapsible-header {
                 background: #312b3b;
                 border-color: #433c4f;
             }
             
-            .dog-calc-collapsible-header:hover {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-collapsible-header:hover {
                 background: #3a3446;
             }
             
-            .dog-calc-section h2,
-            .dog-calc-collapsible-header h3,
-            .dog-calc-form-group label,
-            .dog-calc-result-label {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-section h2,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-collapsible-header h3,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group label,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-result-label {
                 color: #f5f3f7;
             }
+
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-unit-label {
+                color: #b8b0c2;
+            }
+
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-unit-label.active {
+                color: #f5f3f7;
+            }
+
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-slider {
+                background-color: #433c4f;
+            }
             
-            .dog-calc-form-group select,
-            .dog-calc-form-group input[type="number"],
-            .dog-calc-form-group input[type="text"] {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group select,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group input[type="number"],
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group input[type="text"] {
                 background-color: #312b3b;
                 border-color: #433c4f;
                 color: #f5f3f7;
                 background-image: url("data:image/svg+xml;charset=UTF-8,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%23f5f3f7' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3e%3cpolyline points='6 9 12 15 18 9'%3e%3c/polyline%3e%3c/svg%3e");
             }
             
-            .dog-calc-form-group select option {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group select option {
                 background-color: #312b3b;
                 color: #f5f3f7;
             }
             
-            .dog-calc-form-group select:focus,
-            .dog-calc-form-group input[type="number"]:focus,
-            .dog-calc-form-group input[type="text"]:focus {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group select:focus,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group input[type="number"]:focus,
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group input[type="text"]:focus {
                 background-color: #312b3b;
                 border-color: #f19a5f;
             }
             
-            .dog-calc-form-group input[readonly] {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-form-group input[readonly] {
                 background-color: #433c4f;
                 color: #b8b0c2;
             }
             
-            .dog-calc-results {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-results {
                 background: linear-gradient(135deg, rgba(241, 154, 95, 0.15) 0%, rgba(241, 154, 95, 0.08) 100%);
                 border-color: rgba(241, 154, 95, 0.3);
             }
             
-            .dog-calc-result-value {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-result-value {
                 color: #f5f3f7;
                 background: rgba(241, 154, 95, 0.2);
             }
             
-            .dog-calc-footer {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-footer {
                 background: #24202d;
                 border-color: #433c4f;
             }
 
-            .dog-calc-unit-label {
-                color: #b8b0c2;
-            }
-
-            .dog-calc-unit-label.active {
-                color: #f5f3f7;
-            }
-
-            .dog-calc-slider {
-                background-color: #433c4f;
-            }
-
-            .dog-calc-action-buttons {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-action-buttons {
                 background: #312b3b;
                 border-color: #433c4f;
             }
 
-            .dog-calc-btn {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-btn {
                 background: #433c4f;
                 border-color: #433c4f;
                 color: #f5f3f7;
             }
 
-            .dog-calc-btn:hover {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-btn:hover {
                 background: #524a5f;
                 border-color: #524a5f;
             }
 
-            .dog-calc-btn-share:hover {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-btn-share:hover {
                 border-color: #9f5999;
                 color: #f19a5f;
             }
 
-            .dog-calc-btn-embed:hover {
+            .dog-calc-widget.dog-calc-theme-system .dog-calc-btn-embed:hover {
                 border-color: #7fa464;
                 color: #7fa464;
             }
@@ -622,16 +736,18 @@
 
     // Calculator class
     class DogCalorieCalculatorWidget {
-        constructor(container) {
+        constructor(container, options = {}) {
             this.container = container;
             this.currentMER = 0;
             this.isImperial = false;
+            this.theme = options.theme || 'system'; // 'light', 'dark', 'system'
             this.init();
         }
 
         init() {
             this.injectStyles();
             this.injectHTML();
+            this.applyTheme();
             this.bindEvents();
             this.updateUnitLabels();
         }
@@ -649,6 +765,17 @@
             this.container.innerHTML = widgetTemplate;
         }
 
+        applyTheme() {
+            const widget = this.container.querySelector('.dog-calc-widget');
+            if (!widget) return;
+
+            // Remove existing theme classes
+            widget.classList.remove('dog-calc-theme-light', 'dog-calc-theme-dark', 'dog-calc-theme-system');
+            
+            // Apply new theme class
+            widget.classList.add(`dog-calc-theme-${this.theme}`);
+        }
+
         bindEvents() {
             const weightInput = this.container.querySelector('#weight');
             const dogTypeSelect = this.container.querySelector('#dogType');
@@ -956,7 +1083,8 @@
         const containers = document.querySelectorAll('#dog-calorie-calculator, .dog-calorie-calculator');
         containers.forEach(container => {
             if (!container.dataset.initialized) {
-                new DogCalorieCalculatorWidget(container);
+                const theme = container.dataset.theme || 'system';
+                new DogCalorieCalculatorWidget(container, { theme });
                 container.dataset.initialized = 'true';
             }
         });