[Feature request] Support installation of color themes under /usr/share/kicad/themes

Currently only themes under $XDG_CONFIG_HOME/kicad/$VERSION/colors are read by the application. It'd be nice if a directory under /, like /usr/share/kicad/themes was supported. This would have the benefit of allowing the packaging of color themes for distros as the home directory shouldn't be touched by package managers. Futhermore, it would allow the global installation of said themes. While this is a very niche use-case it shouldn't be too hard to implement.

KiCad Version

Application: KiCad x86_64 on x86_64

Version: 7.0.8+1, release build

Libraries:
	wxWidgets 3.2.2
	FreeType 2.12.1
	HarfBuzz 6.0.0
	FontConfig 2.14.1
	libcurl/7.88.1 OpenSSL/3.0.11 zlib/1.2.13 brotli/1.0.9 zstd/1.5.5 libidn2/2.3.3 libpsl/0.21.2 (+libidn2/2.3.3) libssh2/1.10.0 nghttp2/1.52.0 librtmp/2.3 OpenLDAP/2.5.13

Platform: Debian GNU/Linux 12 (bookworm), 64 bit, Little endian, wxGTK, gnome, wayland

Build Info:
	Date: Oct 13 2023 18:07:56
	wxWidgets: 3.2.2 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.6.3
	Curl: 7.88.1
	ngspice: 39
	Compiler: GCC 12.2.0 with C++ ABI 1017

Build settings:
	KICAD_SPICE=ON