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.