I'm surprised I can't find any existing thread on this, but is there any tutorial or info on creating custom themes? The accompanying config files are rather incomprehensible and each theme has a different number of varying graphics, so modifying an existing theme without context is not exactly possible.

There isn't any such tutorial, nor was one ever envisaged. A new 'Theme Builder', a GUI based tool for creating themes, which guides users through the process step by step, will eventually come along if there is sufficient demand.

Creating themes from scratch, or even modifying an existing one, by editing the config files (and providing the appropriate gfx) is something that could quickly get anybody into trouble unless they are an experienced user and know what they are doing.

The problem is also that theming or skinning has really lost much if not most of its appeal generally in the last decade-plus - the number of apps that support skinning/theming that are still actively developed is a good indicator of this. Apart from Winstep, there are next to none.