If you want to apply different styling to your websites and you achieve this by switching to different css front-end frameworks you're going to have a tough time after a few websites. You will not only have to learn how to use these frameworks every time, but also after a few websites you're going to run out of frameworks to try out.
Instead, why not work out how to customize Bootstrap such that you can make it look unique (I know that Bootstrap out of the box is becoming ubiquitous and it's nice to look different).
There are some nice ideas here about customizing Bootstrap and there are other resources out there too if you look for them.
Get good at using one front-end framework and understand how to customize it and you'll be a more productive developer as a result.