@leocg OK thank you... I found an earlier thread on this issue from 2019 and quite a lot of agreement to change that color or give several alt-options. I read your comment that "this is the color the designers chose".

So no options in flags to modify? I presume not,

Using a Chrome extension with Slimjet (GetCRX), I can pack installers for extensions and themes from their Chrome pages. The extension installers so packed will install into Opera. Opera refuses to install the themes, however.

I don't think they will add anything new in the near future.