There are a lot of great resources to help you create better CSS code.
I will add on to this article when I find additional resources that I find useful.
Image effects and image hover effects
Hover over buttons and create an icon effect:
Picks up colors from web sites with stylifyme.
designyourway – material design resources for designers
Long lists of great resources to go through:
designyourway – the most popular css libraries that are on github
Simplilearn – css3 resources ultimate list article
22 Essential CSS Recipes