You could put a giant indicator of who you are in your sig like me. Or you could have your normal username as your personal text. There's a number of ways you can signal your identity without your username. I do admit I miss back when people's default username appeared below their pfp and the changed name was only a display name, but that was before you joined.
Super Mario Boards
Czario
zel
people do constantly have themes on here and its so common i dont have much trouble (though i do admit i have to check every time who this user with a new theme is)
Cell
Some of my topics refer to me "Miss Pinky', that flat out was never a username I used and people still know it's me, it will be fine