Linux Issues (Various)

[OdinVex]

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

[OdinVex]

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).