Hey Larsgj,
APDD’s suggestion should work if the issue is ultimately that your custom.css file’s style code is being overridden by CSS code elsewhere. (Whether that’s the case can be discovered through using Firebug or the inspect element tool.)
I’d recommend also double-checking that custom.css is actually referenced on the page with the form on it!
The other approach to the issue (if it is a CSS overriding issue and you don’t want to use !important), is assessing why the CSS is being overridden, then resolve it that way. If it involves CSS hierarchies, you’ll need to revisit how CSS hierarchies work and see how those overrides/priorities can be addressed by changing around the way you set that CSS code.
Thanks for the reply APDD! Much appreciated. ??
If you have any other questions, feel free to post back here.