Like most things, it depends on the project. At work, I'm working on a project that's going down the BEM path (with a 7-1 Architecture pattern). I've done this with a few personal projects. I'm currently working on a side project that uses Tachyons, and it's nice (I even made my own module). I'm using it because I'm interested in the DRY-ness of functional CSS. The downside is, of course, that there's a very real chance my HTML will be less DRY -- but that's what templates are for.
Regardless, for all custom CSS I make sure to have settings defined away from styling - things like spacing units, font stacks & sizing, colors, etc. These should be consistent with whatever framework you're using if you choose to use one.
I don't use frameworks very often, but I do use Sass libraries like Bourbon
I use rems for as many things as possible, with ems for media queries (as they're most consistent between browsers). Of course, there are small exceptions here and there.
As far as naming patterns go though, the most important thing is consistency. So I wouldn't mix BEM & Bootstrap -- I'd follow their naming convention.
Like most things, it depends on the project. At work, I'm working on a project that's going down the BEM path (with a 7-1 Architecture pattern). I've done this with a few personal projects. I'm currently working on a side project that uses Tachyons, and it's nice (I even made my own module). I'm using it because I'm interested in the DRY-ness of functional CSS. The downside is, of course, that there's a very real chance my HTML will be less DRY -- but that's what templates are for.
Regardless, for all custom CSS I make sure to have settings defined away from styling - things like spacing units, font stacks & sizing, colors, etc. These should be consistent with whatever framework you're using if you choose to use one.
rems for as many things as possible, withems for media queries (as they're most consistent between browsers). Of course, there are small exceptions here and there.