Add theme support for customizable colors

[FlipFlopszzz]

Add theme support for customizable colors, format: RGB/RGBA

Example config

{
"theme": {
  "black": "#000000",
  "red": "#FF0000FF",
  "green": "#00FF00",
  "yellow": "#FFFF00FF",
  "blue": "#204DBE",
  "magenta": "#FF00FFFF",
  "cyan": "#00A7B2",
  "white": "#FFFFFFFF",
  "brightBlack": "#808080",
  "brightRed": "#FF3E30FF",
  "brightGreen": "#58EA51",
  "brightYellow": "#FFC944FF",
  "brightBlue": "#2F6AFF",
  "brightMagenta": "#FC74FFFF",
  "brightCyan": "#00E1F0",
  "brightWhite": "#FFFFFFFF",
  "background": "#0C0C0C",
  "foreground": "#00FF00FF",
  "lineNumber": "#FF00FF",
  "lineHighlight": "#00FF00FF"
}
}

Scope

This is a lightweight change with no new dependencies.

The changes are limited to:

• settings.rs: Theme config parsing and storage
• documents.rs: Apply theme colors when creating buffers
• buffer/mod.rs: TextBuffer color setters
• main.rs: Apply user theme colors to indexed colors array
• framebuffer.rs: Fix background/foreground color application from indexed colors

Validation

cargo test -q

Notes

• All theme fields are optional; omitted colors fall back to defaults
• The theme configuration reuses the existing settings.json
• Binary size increase 5KB (+1.3%) (measured with cargo build --release)

Only tested on Windows 11 with cargo 1.86.0

Screenshot

[FlipFlopszzz]

@microsoft-github-policy-service agree

[lhecker]

Unfortunately, I'm not sure if we can merge this. My vision was to introduce a CSS-like language for theming of the application. I briefly mention this here:

edit/crates/edit/src/tui.rs

Lines 86 to 87 in 7c0b3d1

	//! The reason "classnames" are used is because I was hoping to add theming
	//! in the future with a syntax similar to CSS (simplified, however).

A while ago I had started a prototype in this branch: https://github.com/microsoft/edit/tree/dev/lhecker/css

[FlipFlopszzz]

I honestly just need a way to customize colors, don't care about the implementation details. Your CSS plan definitely sounds more robust.
If the CSS plan is still a ways off, this PR might help bridge the gap in the meantime.

[unoukujou]

It needs to have also ability to theme the menu bar and status bar and the file open dialog... etc

Whole interface needs to theme for achieving the best result when you want it to match and blend together with all your other TUI apps.