I'm so annoyed by modern UI trends, and that they're copied everywhere, including Linux desktops.
For example, in Windows 10 (maybe already in 8?) by default you can hardly tell the focused Window apart from other Windows, because the main difference is that the window title text is dark grey instead of light grey or something like that?
Win10 lets you choose an "accent color" and has a setting to apply that to window titles and window borders (which otherwise are basically invisible) - but that only changes the window title color for some (more classic) Windows applications, more modern ones (of course including several Windows builtin settings apps etc, or Visual Studio) don't use the accent color for the window title bar.. this probably is one of these mobile designs (doesn't matter if apps are fullscreen, or even if they're not it's less annoying if you only have one screen), but how could this happen, do Microsoft developers not use their own software? Or do they only use one screen? Or did they realize that it's unergonomic and someone forced them to follow the design anyway?
Anyway, nowadays many (most?) XFCE window manager themes, including the default one and the widely used Greybird theme, do the same thing (focused window looking mostly the same as unfocused ones). And, based on some Gtk3 fad introduced by GNOME (client side decorations), some of the XFCE apps don't even use the window manager window title bar, but ones that somehow come with the Gtk theme - so even if you use one of the XFCE themes that do have readable window title bars, those apps don't (unless you use hacks like gtk3-nocsd). Yes, they took the one thing away that gave graphical Linux applications (with different toolkits) at least a bit of a unified look..
(Not sure about KDE, but GNOME window manager themes will certainly have the same problem as XFCE, presumably worse)
And then there's all the problems with flat low-contrast UIs, like tabs where it's hard to tell which one is active, and for the remaining tabs the only way to tell where one starts/ends is their icon or close button, scroll bars where the thumb is grey on a slightler lighter grey background (or scrollbars that hide themselves, the single least useful feature ever for DESKTOPS THAT USE WIDESCREEN DISPLAYS AND THUS HAVE PLENTY SPACE FOR (VERTICAL) SCROLLBARS), etc
Maybe desktop UI really did peak with Windows 2000.
IIRC just because Apple at some point overdid it with the Skeuomorphisms and it looked a bit silly, they (and then everyone else) went to the other extreme of bland flat UIs