diff --git a/src/App.css b/src/App.css
new file mode 100644
index 0000000..f40c522
--- /dev/null
+++ b/src/App.css
@@ -0,0 +1,33 @@
+/* Accessibility Focus Styles */
+button:focus-visible,
+textarea:focus-visible,
+input:focus-visible {
+  outline: 3px solid #ffa500;
+  outline-offset: 2px;
+}
+
+/* Button hover and active states */
+button:hover {
+  background-color: #006a8e !important;
+  transform: translateY(-1px);
+}
+
+button:active {
+  transform: translateY(0);
+}
+
+/* High contrast mode support */
+@media (prefers-contrast: high) {
+  button,
+  textarea {
+    border-width: 3px;
+  }
+}
+
+/* Reduced motion support */
+@media (prefers-reduced-motion: reduce) {
+  button,
+  * {
+    transition: none !important;
+  }
+}
diff --git a/src/App.tsx b/src/App.tsx
index 851cc82..faba58a 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -1,46 +1,129 @@
 import { useState } from 'react';
+import { add } from './stringCalculator';
+import './App.css';
 
 const App = () => {
   const [input, setInput] = useState('');
-  const [result] = useState(null);
+  const [result, setResult] = useState<number | null>(null);
+  const [error, setError] = useState<string>('');
 
-  const handleCalculate = () => {};
+  const handleCalculate = () => {
+    try {
+      setError('');
+      const sum = add(input);
+      setResult(sum);
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'An error occurred');
+      setResult(null);
+    }
+  };
+
+  const handleInputChange = (e: React.ChangeEvent<HTMLTextAreaElement>) => {
+    setInput(e.target.value);
+    setError('');
+    setResult(null);
+  };
 
   return (
-    <div style={{ padding: '20px', backgroundColor: '#fff', color: '#aaa' }}>
+    <div style={{ padding: '20px', maxWidth: '800px', margin: '0 auto', backgroundColor: '#fff', color: '#333' }}>
       <img
         src='https://images.unsplash.com/photo-1594352161389-11756265d1b5?q=80&w=2574&auto=format&fit=crop&ixlib=rb-4.1.0&ixid=M3wxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D'
         width={600}
         height={400}
+        alt='Calculator with colorful number buttons and mathematical symbols'
+        style={{ display: 'block', maxWidth: '100%', height: 'auto', marginBottom: '20px' }}
       />
 
-      <h2>String Calculator</h2>
+      <h1 style={{ fontSize: '32px', marginBottom: '10px', color: '#333' }}>String Calculator</h1>
 
-      <h1 style={{ fontSize: '20px' }}>Enter numbers</h1>
+      <p style={{ fontSize: '18px', marginBottom: '20px', color: '#555' }}>
+        Enter numbers below to calculate their sum
+      </p>
 
+      <label 
+        htmlFor='numbers-input' 
+        style={{ display: 'block', marginBottom: '8px', fontWeight: 'bold', fontSize: '16px', color: '#333' }}
+      >
+        Numbers to calculate
+      </label>
       <textarea
-        style={{ margin: '10px 0', color: '#aaa' }}
-        placeholder='Enter numbers'
+        id='numbers-input'
+        style={{ 
+          margin: '0', 
+          color: '#333',
+          padding: '10px',
+          width: '100%',
+          maxWidth: '600px',
+          minHeight: '100px',
+          border: '2px solid #333',
+          borderRadius: '4px',
+          fontSize: '16px',
+          fontFamily: 'inherit',
+          boxSizing: 'border-box'
+        }}
+        placeholder='Enter numbers (e.g., 1,2,3 or use custom delimiters)'
         value={input}
-        onChange={(e) => setInput(e.target.value)}
+        onChange={handleInputChange}
+        aria-describedby='input-help'
       />
+      <p id='input-help' style={{ fontSize: '14px', color: '#555', marginTop: '8px', marginBottom: '15px' }}>
+        You can use commas, newlines, or custom delimiters (e.g., //;\n1;2)
+      </p>
 
-      <div
+      <button
         onClick={handleCalculate}
         style={{
-          padding: '10px',
+          padding: '12px 24px',
           backgroundColor: '#008cba',
           color: '#fff',
-          border: 'none',
-        }}>
+          border: '2px solid #006a8e',
+          borderRadius: '4px',
+          fontSize: '16px',
+          cursor: 'pointer',
+          fontWeight: 'bold',
+          marginTop: '10px',
+          transition: 'background-color 0.2s ease'
+        }}
+        aria-label='Calculate the sum of entered numbers'
+      >
         Calculate
-      </div>
+      </button>
 
-      {result !== null && <p style={{ color: 'green' }}>Result: {result}</p>}
+      {result !== null && (
+        <div 
+          role='status' 
+          aria-live='polite'
+          style={{ 
+            color: '#006400', 
+            fontSize: '20px', 
+            fontWeight: 'bold', 
+            marginTop: '20px',
+            padding: '15px',
+            backgroundColor: '#d4edda',
+            border: '2px solid #006400',
+            borderRadius: '4px'
+          }}
+        >
+          <strong>Result:</strong> {result}
+        </div>
+      )}
 
-      <div role='alert'>
-        <p>Make sure you enter numbers correctly!</p>
-      </div>
+      {error && (
+        <div 
+          role='alert' 
+          aria-live='assertive'
+          style={{ 
+            color: '#d8000c', 
+            backgroundColor: '#ffd2d2', 
+            padding: '15px', 
+            marginTop: '20px',
+            border: '2px solid #d8000c',
+            borderRadius: '4px'
+          }}
+        >
+          <strong>Error:</strong> {error}
+        </div>
+      )}
     </div>
   );
 };
diff --git a/src/stringCalculator.test.ts b/src/stringCalculator.test.ts
index e69de29..9bd8175 100644
--- a/src/stringCalculator.test.ts
+++ b/src/stringCalculator.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from 'vitest';
+import { add } from './stringCalculator';
+
+describe('String Calculator', () => {
+  describe('add', () => {
+    it('should return 0 for an empty string', () => {
+      expect(add('')).toBe(0);
+    });
+
+    it('should return the number itself for a single number', () => {
+      expect(add('1')).toBe(1);
+      expect(add('5')).toBe(5);
+    });
+
+    it('should return the sum of two comma-separated numbers', () => {
+      expect(add('1,2')).toBe(3);
+      expect(add('10,20')).toBe(30);
+    });
+
+    it('should handle multiple numbers', () => {
+      expect(add('1,2,3,4,5')).toBe(15);
+    });
+
+    it('should handle newlines as delimiters', () => {
+      expect(add('1\n2,3')).toBe(6);
+    });
+
+    it('should support custom delimiters', () => {
+      expect(add('//;\n1;2')).toBe(3);
+      expect(add('//|\n1|2|3')).toBe(6);
+    });
+
+    it('should throw an error for negative numbers', () => {
+      expect(() => add('1,-2,3')).toThrow('Negative numbers not allowed: -2');
+      expect(() => add('1,-2,-3')).toThrow('Negative numbers not allowed: -2, -3');
+    });
+
+    it('should ignore numbers greater than 1000', () => {
+      expect(add('2,1001')).toBe(2);
+      expect(add('1000,1001,2')).toBe(1002);
+    });
+
+    it('should handle delimiters of any length', () => {
+      expect(add('//[***]\n1***2***3')).toBe(6);
+    });
+
+    it('should handle multiple delimiters', () => {
+      expect(add('//[*][%]\n1*2%3')).toBe(6);
+    });
+
+    it('should handle multiple delimiters of any length', () => {
+      expect(add('//[**][%%]\n1**2%%3')).toBe(6);
+    });
+  });
+});
diff --git a/src/stringCalculator.ts b/src/stringCalculator.ts
index e69de29..808efc2 100644
--- a/src/stringCalculator.ts
+++ b/src/stringCalculator.ts
@@ -0,0 +1,48 @@
+export function add(numbers: string): number {
+  if (numbers === '') {
+    return 0;
+  }
+
+  let delimiter = /[,\n]/;
+  let numberString = numbers;
+
+  // Check for custom delimiter
+  if (numbers.startsWith('//')) {
+    const delimiterEndIndex = numbers.indexOf('\n');
+    const delimiterSection = numbers.substring(2, delimiterEndIndex);
+    numberString = numbers.substring(delimiterEndIndex + 1);
+
+    // Handle multiple delimiters of any length
+    if (delimiterSection.startsWith('[')) {
+      const delimiters = delimiterSection.match(/\[([^\]]+)\]/g);
+      if (delimiters) {
+        const escapedDelimiters = delimiters
+          .map(d => d.slice(1, -1))
+          .map(d => d.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'))
+          .join('|');
+        delimiter = new RegExp(escapedDelimiters);
+      }
+    } else {
+      // Single character delimiter
+      const escapedDelimiter = delimiterSection.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+      delimiter = new RegExp(escapedDelimiter);
+    }
+  }
+
+  const nums = numberString
+    .split(delimiter)
+    .map(n => n.trim())
+    .filter(n => n !== '')
+    .map(Number);
+
+  // Check for negative numbers
+  const negatives = nums.filter(n => n < 0);
+  if (negatives.length > 0) {
+    throw new Error(`Negative numbers not allowed: ${negatives.join(', ')}`);
+  }
+
+  // Sum numbers, ignoring those greater than 1000
+  return nums
+    .filter(n => n <= 1000)
+    .reduce((sum, n) => sum + n, 0);
+}