Which program XFCE uses for managing GTK themes?
I use Gnome and Bless looks great with my dark theme, I'm not really sure where the problem is in this case.
The only thing I had to adjust is the output window theme because it was white, it can be changed under View > Layouts.

I don't know which tool it is exactly, but this is the website help for the theming:
https://docs.xfce.org/xfce/xfce4-settings/4.16/appearance
The command line is xfce4-settings-manager.

Choosing "Adwaita dunkel" (Adwaita dark), the menu is black text on dark background and barely visible. The text in the menus is only visible when I hover over them (see screenshot).
Choosing "Breeze-dark", I get white text on black background for the menu bar (which is at least visible) but the problem in the menu still occurs. And then also the text on the buttons is white and therefore unreadable.

It looks like some elements are skinned differently than others. The text color changes, but not as it should.
Choosing "Breeze" or "Adwaita" (the non dark themes) text and background are correct.

I just noticed that I have the same problem with monodevelop, so maybe it's a problem within mono (+xfce) and not bless specific.
But Ryujinx, which is also written in C#, works just fine.

In GNOME Builder if I select Style = "System" it displays the code in the light variant. So that may be part of the confusion - not really detecting the system settings. The UI however is rendered correctly (dark).

FWIW I opend a bug report at https://gitlab.xfce.org/xfce/xfce4-settings/-/issues/356

Runnung GTK2_RC_FILES=/usr/share/themes/Adwaita-dark/gtk-2.0/gtkrc bless works for me.