Add WindowSkin: let a separate plugin theme the settings window

[MikalaiKryvusha]

What

A small extension point so that a separate plugin can give the settings window the look of the game it is used in. The plugin itself ships no themes and its default look does not change.

• ConfigurationManager.WindowSkin (GUISkin, null by default). When set, the window, its controls and tooltips are drawn with it, and its window style draws the window background instead of the built-in grey one. The dimmed backdrop behind a window that has not been moved yet keeps the default skin.
• Plugin and category headers use the skin's custom styles named ConfigurationManager.PluginHeader and ConfigurationManager.CategoryHeader (constants PluginHeaderStyleName / CategoryHeaderStyleName) if the skin has them. Otherwise they are built from the skin's label as before. The private fields _pluginHeaderSkin / _categoryHeaderSkin keep their names, so plugins that already reach into them keep working.

Usage from a theme plugin:

var manager = (ConfigurationManager.ConfigurationManager)Chainloader.PluginInfos[ConfigurationManager.ConfigurationManager.GUID].Instance;
manager.WindowSkin = myGameSkin; // optionally with customStyles named ConfigurationManager.PluginHeader / CategoryHeader

Why

Mods that want the window to match their game currently have to patch it with Harmony: replace GUI.skin in a prefix to OnGUI, force _windowWasMoved so that the grey background is not drawn under a themed window, and restyle the header styles through reflection. That breaks whenever the internals change. With WindowSkin the theme stays in its own plugin and needs no patches. Separate from #125 (translation): this PR touches only the skin handling.

Testing

Unity 2020.3 Mono game, BepInEx 5.4.23.5. The theme plugin is the game's own UI mod: dark paper and old gold. It uses WindowSkin when the property exists and falls back to its old Harmony path otherwise.

• Default look, unchanged (WindowSkin null):

  

• WindowSkin set by the theme plugin, headers from its named custom styles:

  

• The same in the game:

  

• Turning the theme off at runtime (setting WindowSkin back to null) returns the default look immediately.

• 19.0 with the same theme plugin, through its old Harmony path, looks the same as before.

• Builds: ConfigurationManager (BepInEx 5, net35) and ConfigurationManager.IL2CPP, 0 errors. Not run in an IL2CPP game.

(The window in the screenshots is in Russian because of #125's translation file; this PR does not depend on it.)

🤖 Generated with Claude Code

[ManlyMarco]

Let me know when you're done working on this plugins and I'll make a release.

[MikalaiKryvusha]

Thank you so much for the quick and thoughtful reviews, and for merging both PRs! I was really glad to help. I'm done working on this plugin, so feel free to make the release whenever it suits you.

[MikalaiKryvusha]

Sorry, one tiny last fix after all: #127 (the slider value box was a fixed 50px and clipped numbers with a bigger font). After that, that's really everything from me. Huge thanks again! We'll be happy to download the release with these changes.