is it possible to let users define/customize their own stylesheets?

I have a blog feature on part of my site which essentially is just a
Posts and Comments controller. I was wondering if it's possible to
let users define or customize their own stylesheets for their blogs
similar to how or Anyone know how?

Sure. Of how depends on exactly what you want to do.

If you want to let users select from several pre-defined themes, you
can define those themes as css stylesheets, and have the
controllers/views (most likely in your layout views) include the
selected stylesheet(s) needed to implement the selected theme. Typo
does this.

If you want to actually let users customize individual css styles,
then you might generate one or more html style tags in a template
using information stored for the user in the database.