To complement and update the response, other plugins
Helion plugin
import the Helion plugin to your website
 https://github.com/geuis/helium-css
<script type="text/javascript" src="helium.js" onload="helium.init()" async></script>
A textbox like this will appear
 just click on START and it will scan all the CSS on your site.
just click on START and it will scan all the CSS on your site.
With this you can analyze that :
- Green selectors are not being used
- Black selectors are being used
- Selectors in blue color, could not be tested due to them only
are activated with some user action, so test manually
afterward.
- Selectors in red color are poorly formed and damaging your CSS,
so make correction or delete them.
 
Chrome Plugin
install the 
https://chrome.google.com/webstore/detail/unusedcss/dokggbghedajooenkgjbamikfgnngeik
Another Chrome plugin, untested but interesting is:
https://chrome.google.com/webstore/detail/css-remove-and-combine/cdfmaaeapjmacolkojefhfollmphonoh
It displays all the unused css and makes it possible to download a new CSS with only the items used, simply replacing yours. but as in Helion should beware of the : hovers of life