Try it first without the use of "!important". If you are inserting this at the bottom of the 'additional.css' template it should not be needed. If a modification loads it's CSS after 'additional.css' it would be required to override the modification, but this might hinder other postbit modifications one might load in the future. Please note @Ozzy47 is not wrong in his answer, as his answer has the best chance in working. Many users do not want a lecture on theory along with a working answer.