Scenario: You have a preinstalled theme from your system and you like it, but you want to change a single color. For example, I like the breeze dark theme, to have good OS integration, but I want to have a darker background of the document. Unless the theme is accidentally available as an extension or downloadable theme, I cant reuse it and need to manually recreate it. A button to copy the used theme into a place like "themename-custom" would be great, and I think multiple applications have such a system. https://bugs.documentfoundation.org/show_bug.cgi?id=165232
Sounds desirable but makes the UI heavier for not much benefit. You should just change the color you don't like - that what the customization is for. Export and import of a whole theme is supposed to be done by people/developers who create new themes.
-1, including theme design into the UI is not necessary. Greatly prefer the LibreOffice appearance themes be curated and maintained as extensions. Users sufficiently motivated to roll their own might use an appropriate tool, e.g. @Rafael's ThemeCreator package, which could probably be posted to Extensions server.
I found out a way to create theme extensions, which seems to be near undocumented? It is pretty hacky, involves extracting and reformatting an XML file. I think it is already possible to change a single color of a theme, but the UX is not so clear. I think an improvement could be useful but it might involve a bigger structural change? Like, any user theme could be saved like an extension, to make redistribution easier. But I leave that up to you.
The topic was on the agenda of the design meeting. To keep the UI simple we should stick to the original idea and do not add import/export interactions. However, likewise the export of custom colors [1] we may provide an extension that does the grunt work of extracting the data from the registry. Neil, something for you? [1] https://extensions.libreoffice.org/en/extensions/show/custom-palette-eport