You can start by using plain, semantic HTML and grabbing a classless CSS with a license you like. (Check out this list.)
It’s good enough for a simple app or site, and it’s honestly impressive how good something can look with just this. It’s the “plain t-shirt and jeans” of web design.
Same thing on my project. Thousands of lines across a few dozen files copied 100+ times. At that point there’s almost no going back with everything diverging so long ago.