[Bug 55771] uxtheme:system - test_ShouldSystemUseDarkMode() fails on Windows 7 to 10 1909

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=55771

--- Comment #6 from Helix Graziani  ---
I think that checking if the registry value exists is a good approach.

When testing the original commit, I deleted the registry values and looked at
the theme of the taskbar.
What I didn't realize was that the registry value being deleted is not a normal
state, as shown in comment 3, where you can see that SystemUsesLightTheme is
set (indicated by the test working when the theme is dark).

So the patch is to skip if the registry value doesn't exist.

(We could also add a version check to see if it's version 1909 and set the
default value accordingly, but since it works on all the other tests and would
just skip, we can leave it.)