Simplify color personalization

[guerinclement]

Issue: Improve Color Palette Customization and Readability

Problem Description

Color palettes in Shapash are currently difficult to customize and understand. The mapping between dictionary keys and the actual HTML elements rendered in the web application is unclear, making personalization unintuitive and error-prone.

For example, in shapash/shapash/style/colors.json (lines 47–51, commit 5c99856):

"featureimp_bar": {
  "1": "rgba(0, 154, 203, 1)",
  "2": "rgba(223, 103, 0, 0.8)",
  "3": "rgba(240, 195, 162, 0.8)",
  "4": "rgba(245, 122, 0, 0.8)"
}

Using numeric keys such as "1", "2", "3" does not convey any semantic meaning, making it hard to determine which color corresponds to which UI element.

Proposed Solution

1. Replace generic numeric keys with explicit, descriptive names wherever possible (e.g. primary, secondary, highlight, background, etc.) to clearly indicate their purpose.
2. Add support for standard color scales (e.g. viridis, plasma, inferno, etc.) to allow easier and more flexible customization of color themes.

Expected Benefits

• Improved readability and maintainability of color configuration files
• Easier customization for users
• Better alignment with common visualization practices