Feature Request: Customizable Colors and CLI Toggles

Hi! First of all, Hibiki looks really nice. I've been using it as a keystroke visualizer on Hyprland, and I really like the overall aesthetic.

I have a couple of feature requests/questions:

### 1. Custom background color

Would it be possible to customize the background color of the keystroke/bubble windows?

I noticed that font family, weight, and size can already be customized, which is great. It would be nice to have a similar level of customization for the background and text colors as well.

For example, something along the lines of:

```toml
[keystroke]
background = "#1e1e2e"
foreground = "#cdd6f4"
```

The exact configuration format is, of course, up to you.

### 2. CLI commands for toggling modes

Would it be possible to expose the toggle actions through the CLI?

Something like:

```bash
hibiki --keystroke-toggle
hibiki --bubble-toggle
```

This would make Hibiki much easier to integrate with compositor keybindings.

For example, on Hyprland I could have:

```ini
bind = SUPER, K, exec, hibiki --keystroke-toggle
bind = SUPER, B, exec, hibiki --bubble-toggle
```

This would also make it possible to control Hibiki without interacting with the system tray or settings UI.

I'm mainly interested in using Hibiki as a lightweight, compositor-integrated key caster, so having CLI-accessible controls would be really useful.

Thanks for the great project!

Start by tracing the existing font customization in the [keystroke] configuration and the toggle actions exposed through the system tray or settings UI. Define how background and foreground colors should be configured, then add CLI entry points for keystroke and bubble toggles. Done means both color settings and compositor-friendly commands work without relying on the tray or settings UI.

Written by the indexing model from the issue text.