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I agree with this - I would like to be able to sort my themes by presets, custom darks, custom lights, downloaded darks, downloaded lights, etc. I personally don't have many themes but I know that many users do, and so being able to sort them as such would be fantastic.
Also, another idea is a list view option of users' themes (a list similar to the Ctrl-Tab "Display Tab Cycler as List") that possibly previews when hovering, similar to how hovering over tabs can display a thumbnail preview. Another sub-idea of the list; folders, so I can have groups of Vivaldi Themes separated as described before, just as how there are folders in Bookmarks, Notes and such.
Small stuff, but it's all appreciated, Team V. Keep chuggin' along, we are very grateful and hope you continue to improve the already feature-rich browser you have made!
Edit: Wow, I didn't realize this was a four year old feature request. I'm glad somebody already had the idea though, this is just bringing it up again.