+1 I’d like this as well. I think overriding themes can also work on a global vs local scope, e.g.
A “default” obsidian theme for all my vaults, e.g. in ~/.config/obsidian.css that I can override locally in each vault e.g. ~/vault/.obsidian/obsidian.css.
It probably makes sense that the global obsidian.css is still different from the theme I want to modify.