This patch makes it so that the theme color is used for the
initial/loading Blender window background color. Unlike what
it may look like, theme settings are well loaded at this point
in the initialization process, which lets us directly use them
for coloring the window background instead of relying on a
default grey color.
This also makes the colored titlebar window decorations (introduced in
#123982 for macOS, #134054 for Windows 11), always blend with the window
instead of looking somewhat broken during window initialization. On the
technical side, this uses the top-bar header background theme color to
ensure this always matches with the color used by the colored titlebar.
Pull Request: https://projects.blender.org/blender/blender/pulls/137425
81efb13f49