overlay tab is only used for DVD playback etc not for regular windows, if theres no default button, uninstalling and reinstalling will set it to the default, but i doubt itl fix your monitors problem
if the monitor is getting on in age , it may simply be dying, there is an internal setting you can turn up as the monitors life shortens, however its temporary (how long depends on how bad) and the insides of monitors are *extremely* dangerous, so unless you are familiar with dealing with 24Kv type power, i'd leave it be, otherwise on the HT transformer theres usually two settings, top is focus and lower is HT level, thats the one you want. it'll brighten up the display. if you dont know what i am talking about, leave it be
and if you do play with it mae sure its unplugged and discharge the anode cap at least twice with 5 minutes in between.
is it dark on the bios or boot screens? if it is, its not windows doing it.