Hi lol just stumbled upon ur acc and thought I'd help u out. Theme is uh...idk how to explain but its just the way your acc is decorated/ designed. For eg: my acc is all purple so you'll say I have a purple theme. So ppl are just complimenting ur acc's style, just thank them haha.
P.S: Always tag the person when replying to them on here otherwise they won't get any notification and think that u didn't answer. If u don't know how to do that, just text me :)