⬇️🟢𝐌 Write styles for light theme 🎨🌞

[Kryptos-FR]

We are using Avalonia's Fluent theme. This them already defines two variants: dark and light.

A number of resource keys (mostly colors) are defined for this theme, with different values for each variant. Our own custom controls and styles should reuse those resource keys for consistency as well as adapting to the user preferred variant.

This task is for the light variant.

See also #2817.

[Kryptos-FR]

Useful links:

• https://docs.avaloniaui.net/docs/basics/user-interface/styling/themes/fluent https://theme.xaml.live/
• https://docs.avaloniaui.net/docs/basics/user-interface/styling/themes/simple
• https://docs.avaloniaui.net/docs/guides/styles-and-resources/how-to-use-theme-variants
• How to add custom theme variants to ThemeDictionaries? AvaloniaUI/Avalonia#11953