Ah I see, you really mean [BETA] 1.5.6.3 but this one killed my Windows 10 twice these days. Black screen with curser after reboot.
May be together with UxStyle_0242_x86_x64_preview installed.
If I load a working theme in WSB [BETA] 1.5.6.3 and just tick on the "test" button all Explorer windows including the desktop change instantly to bold letters.
Un-checking "test", changing theme to a default one, reboot - the bold letters remain forever...
p.s. -> has something to do with Windows running on a 4K screen (3840x2160). This is only practicable if Windows is entirely set to 150%.