Rather than needing to overwrite obsidian.css to change themes, it would be nice if we could store many themes - all with intelligible names - within one folder and then point to the one that we want to use from within Obsidian settings.
An alternative is to not filter out .css files, so that there could be a CSS folder within the vault, each with a theme’s subfolder, and then you drag and drop each theme’s obsidian.css file to the vault root.