Ok, I fixed the issue of not enabling medium size images of folders in Categories view...Here is the solution:
After installing the theme, the setting would default to "small with shadow" in Global Parameters. I changed those, but I did not see any change in the gallery's display.
The problem was that the setting in the menu link reverted from "Use Global" to the specific one that had been set after installing the theme. Actually, the same went for most if not all settings related to the image display in Category View.
I reinstalled the "transparent" theme - which is not at all transparent - configured it, but I really detest the changed icons for back buttons and especially for the download links and the details...
how can I revert to the previous icons, the ones Phoca Gallery had before I installed any themes (installing the default theme again, simply gave me another set of icons, different from the original) ?
I am talking about these icons
https://www.phoca.cz/demo/