Normalize and improve font sizes

This commit standardizes font sizes across components for a uniform
look. The icon sizes, font weights and line heights are also adjusted
accordingly for better standardization and simplicity.

- Introduce variables for standard font sizes, enhancing
  maintainability.
- Remove explicit pixel values, replaced with scalable units based on
  root size.
- Remove workaround for line-height adoptation of bigger font-size.
- Use consistent small font-size for the code area.
- Adjust checkbox tick to scale with font size.

src/presentation/index.html

@@ -41,11 +41,9 @@
         background: #eceef1;
         margin: 5rem auto;
         max-width: 800px;
-        font-size: 7px;
         padding: 3rem;
         border: 1px solid#333a45;
-        font-size: 1.5rem;
-        line-height: 150%;
+        font-size: 18px;
         font-family: 'Monaco', 'Menlo', 'Ubuntu Mono', 'Consolas', 'source-code-pro', monospace;
       }
     </style>