Hi lemonpath,
Yes,Why don’t you try removing this Custom CSS too? You can make a backup of it, remove it and test it. There may be some conflict with plugins too. So you need to deactivate the plugins and check whether your problem solves or not. Deactivating plugins, i don’t think you will lose some thing. If deactivating also doesnot works then reactivate the same plugins.
For the test method just download a clean-retina-pro theme and unzip the folder. change the filename of it and also change the theme name on style.css too. Again zip that theme and install and activate it. If this solves the problem then there is something issues on your current active theme.
As also there may be some scripts on widgets too. So check it and try removing it also. There is no other way than this. You may also try this in your local environment using mamp
(for mac) or xaamp or wamp
(for windows)
Thank you!