An open-source alternative to UltraUXThemePatcher, allows you to install third-party themes.
The patch works by modifying a function in themeui.dll and uxinit.dll to skip a signature check. All credit for the patch goes to M. Hoefs.
Tested on Windows 10 22H2, build 19045.4529. If you want support for another version, open an issue.
Caution
Modifying system files always poses a risk of rendering your system unbootable. Make sure you have an up-to-date restore point, as well as a copy of the original DLLs and another bootable drive in case things go wrong.
python3 patch.py
Then boot into safe mode or an installer USB and replace the two files in %windir%\System32 with the patched copies.