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[Not necessarily a problem, but a general question. I use object editor a lot, and some of the included themes are pretty easy on the eyes, but it pains me having to set the theme each time I close/open an object. Is there a (relatively) easy way to set what “default” theme is selected?]
After renaming to custom.css, you need to update CSS selectors. For example like below
In my case I have converted from “.cm-s-xq-dark” to “.cm-s-custom”. Once you saved CSS file, better to clear the browser cache or open it in new browser or restart the server.