If you go into setting/just settings.xml line 5
<colour name="justbg" value="rgb(0,0,0)"/>
you can change the bolded to higher values
or replace value="rgb(0,0,0)"/> with value="grey 200"/> where the bolded here can be changed in increments of 100 so if 200 was too bright try 300 etc e.g
<colour name="justbg" value="grey 200"/>