Jack-Myth
Linux Issues (Various)
I modified UE4EditorCustomize.uplugin to allow Linux and attempted to compile the plugin, but there were some issues regarding code quality that needed to be addressed just to compile, all with EditorCustomize/Source/UE4EditorCustomize/Private/UE4EditorCustomize.cpp alone. It seems to me that the developer comes from a Windows-only background using old-style C (pre-C++11). See Pull Request #4.
Strings should never use forward-slashes unless they're escaping a character. #include <Editor\UnrealEd\Classes\Factories\FontFileImportFactory.h> should be #include <Editor/UnrealEd/Classes/Factories/FontFileImportFactory.h>. Windows autocorrects this behavior but other platforms don't always do this.
There is a rather confusing code-clock that I removed:
#if ENGINE_MINOR_VERSION >=18 ...return false;... #endif
...The function was a UFontFace*. No idea why they'd try to return false.
Various deletion of arrays using delete instead of delete[]. Fixed.
_constexpr char* UThemeHead = "UTheme"; was changed to _char* UThemeHead = "UTheme"; for now.
Everything now works on Linux on my compile, using Unreal Engine 4.26.2.
One last cosmetic issue that I'm not sure if it is Linux-specific or not is the Menu on every window seems to have its texture offset. Perhaps setting the background to transparent might work, but that affects context menus... shrug o.o
Love the theme, by the way. :) You can delete this issue, I created the PR to fix these issues (except for the menu backgrounds (not context-menu) being offset).