Add Docs for making custom themes.

[AraHaan]

I would like a detailed document that documents (hopefully) all of the color options with an sample image of what the setting changes within the IDE.

My goal of this issue is to help people new to making Visual Studio themes understand and learn what each specific color option that is possible to be set easier.

For my theme, I needed transparency which sadly the theme designer for VS2022 lacked the ability to set a specific transparency value for all of the colors (excluding the document editor text) to be partially transparent which would be helpful for those who use ClaudiaIDE to expand a specific background image throughout the entire IDE.

For example I want to recolor this part but do not know where to look:

cc: @madskristensen