@3dvs
It's even simpler: You don't need to update the themes at all if you like them the way they were when you installed them in your browser. 😃 So theoretically, the theme kind of “auto-updates” with your browser because the theme is just some kind of “setting” for the browser UI and does not necessarily require to be updated to be compatible with the browser or to apply security fixes.
If you download a theme package not for installation but to take a look at how a theme “DNA” looks like (right click → save link as) and extract the zip, you'll notice that themes only contain a background image, sometimes icon files (images) beside a settings.json file.
The settings.json File contains the values the theme editor from the browser itself uses and understands. That's why — no matter how old or new — theme files are also compatible with newer versions of Vivaldi without problems.
If the author changes aspects of the theme file — like images, icons, or colors — the theme itself changes, and you need to re-install it if you want to have those new icons or background images applied. (Or simpler: In the Vivaldi theme editor you'll find an “update theme”-button then, just click it to apply the changed version of the theme) But if you still like the theme that is installed, you basically never need to update it for compatibility reasons. Just for aesthetical reasons, and only if you really want changed aspects of a newer theme version applied to your browser UI.
The fact that you have to trigger this kind of “update” (or, better said, “replacement of settings and graphics”) manually is a good thing! If you alter an installed theme to your needs, it would be bad if the theme gets a new version and overrides your changes every time automatically. Many users, for example, set individual background images or modify the roundess-factor or use different iconsets from another theme.
By the way: You can also change the themes with the editor directly in Vivaldi, and if you use the theme export function there, you'll get a theme package again with a .json file that contains your changes. If you publish it the theme gets an id and the possibility to check the version for a manual update ("theme replacing") then, but for the browser it is irrelevant which version of a theme you are using and I'm sure if any new theme-values sould ever be added to the browser, it will still not break but just use a fallback / default value instead.
