Just installed the beta and I'm in "light mode." My tabs are positioned at the top of the lister.
When I have several tabs open, it is not obvious to me which tab is highlighted or is otherwise indicated to be the one that is active.
I think this changed since Opus 12.
In the attached image, "Downloads" is active.
Default tab colors in light mode are the same as they always were, but can also be changed if you want something that stands out more.
Preferences / Colors and Fonts / Directory Opus Colors / Folder tabs
Edit: (Screenshot was added after I replied.) Those aren't the default colors; looks like they've already been modified.
I modified it per your instruction. I have no recollection of changing them myself, but I must have! Looks good now. Thanks, Leo-