I just use Bootstrap and don't worry about learning CSS. Probably because I suck as CSS.
But if I can use a few Bootstrap classes to make my app 'presentable' and 'professional-looking' and spend my time on what's important...functionality and security...then I'm happy to.
Bootstrap is perfectly fine. I know there's a lot of CSS snobs out there who rail on it but it's a great framework and perfectly acceptable starting point.