Just Use Bootstrap 3
My current thinking (2017)
Actually these days thanks to it being possible to ignore anything prior to IE 11 I find you can get a long way with a few hundred lines of Stylus, a lightweight CSS reset and heavy use of Flexbox. Like I do here.
I know enough CSS to spend hours 'fine tuning' the appearance and responsiveness of web projects. But it is usually a waste of time. Bootstrap 3 gives you a quick and beautiful, mobile first framework that can be easily extended (this website adds a few tweaks). Just use it. Even if you do change your mind (you probably won't) it got you started.
But all Bootstrap websites look the same
Use Bootswatch or other themes. Don't try to create your own (it is almost as bad as doing the CSS by hand (obviously if you do this use LESS, but even this has too many options).
But is really 'heavy weight'
From the get Bootstrap website. Pay attention to the (short and clear) documentation there.
Some thoughts about Internet and web related topics. Design, frameworks, approaches and so on.