Weird. They changed the name. It used to just be tapatalk but now its community reader by tapatalk. Anyway. You bought a themed version of tapatalk. There is the original (my link) which has standard color choices of light and dark and blue orange and green. And then there are the custom color versions that are themed a particular way. You bought the blue and black themed one as opposed to standard tapatalk.
I would email the developer or comment on the blue themed one and complain that there is no true light mode and you want to switch for the standard app. But its up to them technically since you have like a 30 minute return window on apps usually.
If they wont do anything and If you can't live with the colors... Buy the original or the newer tapatalk 4 .. sorry mate

Sent from my ADR6400L using Tapatalk 2