Why is it not possible to opt out of using this new UI engine (for example, some kind of switch in the settings)?
Reason: The entire UI library has been replaced, because the previous one was abandoned by Microsoft decades ago and couldn't be further used because it wasn't flexible enough to add anything new to the interface.
This took over a year for Clickteam to accomplish, to allow dark mode which was a rightly requested feature since forever, and since the new library uses SVG (vector based) icons, the old icons are not compatible.
The UI library in question is a fork of the original library, one that is continuously updated.
But, there is kind of a workaround, which is using a custom skin with a broken setting, forcing it to use the 16bit icons (from before MMF2, something between MMF1.5 and MMF2), I can provide the skin.
One warning is that it causes the close button etc become white on light skin.
Unfortunately the skinning doesn't allow custom icons on toolbars for the moment, but you can use the legacy toolbar customization to make your own icons but limited to a 16 color palette.
Just right click any toolbar icon, choose customize, after it opens a new window, completely ignore that and right click the icon you want to customize from the toolbar itself, then a Button Appearance... option will appear.
Anyway, I'll leave the skin as well if you want to check that.
After copying it to the skins folder, either in Fusion's path or Roaming, you'll need to enable the new Skin toolbar on View menu, and select the custom skin there.