Replace hardcoded rgb values with css variables

Replace hardcoded RGB values with CSS variables for improved code flexibility and maintainability.

## Proposed Changes:

+ Identify and replace all hardcoded RGB values with CSS variables.
## Expected Benefits:

+ Enhanced code flexibility and customization.
+ Improved code maintainability and readability.

## Note:
+ IDA's CSS-based language defines variables using the syntax: `@def variable value;`.
+ Consider defining variables' value using RGB values only, like: `@def colorGreen 64, 160, 43;`. This allows using the same variable for both RGB and RGBA values, e.g., `rgba(${colorGreen}, 0.80)`.
+ Alternatively, you can consider replacing all RGBA functions with the IDA `@lighten` macro for the desired effect.

Start by locating the theme's CSS-based source files and identifying the hardcoded RGB and RGBA values. Read the IDA CSS variable syntax noted in the issue, then decide whether to use RGB variables or the @lighten macro; done means the hardcoded color values are replaced consistently while preserving the theme's appearance.

Written by the indexing model from the issue text.