CiviPledge custom styling
It's best to not edit the files directly, but to create your own file. Otherwise it will get overwritten with each upgrade you do. In the version I'm using, I can set the custom CSS file here:

/civicrm/admin/setting/url?reset=1

I use the "inspect" option in Chrome to see what CSS is being applied, then I copy it into my custom css file, and edit it.