You should attempt to report it as a bug and specify what preset you are using. But as you can see, with a little CSS you can fix this issue in the meantime.
Themes
Boost unstyled quiz navigation buttons
གྲོས་བསྟུན་འདི་བསྡམས་ཡོདཔ་ལས་ ལན་གསལ་བརྐྱབ་མི་ཚུགས་