Bug 455059

SUMMARY
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When downloading new Cursors (themes), some of the cursor sets don't even appear in the available cursors box. 
And those downloaded sets that do appear, about 50-60% of the time, though the name of the theme/set that you just downloaded appears under the box fine, but what you see are exact copies of the Breeze or Breeze Light (so, default) boxes of cursors, and the actual movable cursor copies the respective Breeze styles ...even when the cursor changes forms as per the environment under it, it is at least consistent in its failures to mimic the Breeze styles. This entire description paragraph so far, [with the exception of themes downloaded not appearing in the Available Cursors Area] is new. What I mean is that I have tried and failed (for various reasons that aren't relevant) a few times over the past couple of weeks though I was not at all able to get online for the past 8 days, so actual time trying was about 3 or 4 days. I do apologize for that. But again, irrelevant. The point is that before the 8 day gap, nearly all of the above was not yet part of the bug.  
   So, the original reason was that of the cursor themes that one could download from System Settings > Appearance > Cursors > Get New Cursors, if you could see the set in the available cursors area, which as stated above, did happen but as far as their appearing or not, I believe it may coincide with the current size of your current cursor when you downloaded the theme. 
Now, IF the downloaded cursor theme was available (which as far as I have been able to tell, if, when you downloaded the theme, and it did appear, regardless of anything that follows, it stayed visible, and could be selected with limited **pre-selected** sizes of 24, 36 only sometimes, 48, and 72--as far as I was able to discern), the sizes had to have been adjusted while using one of the Breeze sets, or (at least in Kubuntu 22.04) one of the Oxygen colors which consisted of Blue, White, Yellow, Black, or Zion [shiny black] Oxygen Cursor Sets. But of the 25+ sets that I tried of Cursors, not one would let the Size drop-down button remain active and usable. 
**NOTE: Steps begin once System Settings was opened, then Appearance > Cursors > Get New Cursors Button is selected. **

STEPS TO REPRODUCE
1. From what I tried, ensure you are using any of the 3 sizes [24, 48, or 72] by choosing one of the Original Sets to set the size.
2.  Choose any Cursor Set, download it, making sure it appears among the other available Cursor Sets, Choose & click Apply.
3.  Having tried over 25, I will guess and say "All/Any" Set you have chosen should gray-out/disable the Size button entirely. 

OBSERVED RESULT
Most(All?) Cursor Sets you download and then enable as per above, no matter what you try, you will not get Size button to do anything at all until you change back to one of the original sets listed above. Notice when changing back or even just clicking on one of the original sets (don't even have to Apply, but it won't let you change anything until you do Apply an original) the Size button instantly revives! Now, change to one of the other sizes while in the original type cursor sets, Apply it, then choose another downloaded set: Size greyed-out on you again, right? 

EXPECTED RESULT
What is supposed to happen is when you change to one of the downloaded cursor sets, and enabling (Apply) it, you should now be able to--within the allowed parameters of the new set of cursors, maybe it is 32 or a rare one [but my favorite], a 36 size--that should be it and you can go on your merry way to conquer monsters, algorithms or nested arrays, without even noticing anything more about the cursor set. In a perfect, Microsoft & Google-free world, anyway. 

SOFTWARE/OS VERSIONS
Windows: ----
macOS: -----
Linux/KDE Plasma: Kubuntu 22.04 LTS (x86_64) / KDE Plasma 5.24.5
(available in About System)
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION  
Kernel: 5.15.0-37-generic (64-bit)  
Graphics Platform: X11
Processors: AMD Ryzen 5 3500U with Radeon Vega Mobile Gfx 
Memory: 5.7GiB RAM (total is 8GB)  
Graphics Processor: AMD Radeon Vega 8