style(contentscript): significantly improve popup css robustness

Some websites alter the look of the popup unintentionally by setting CSS properties that are either unset or with a higher specificity than the contentscript before this MR. This can be prevented by setting all: initial !important; Important helps here to further improve the robustness against "outside" properties with high specificity. It also works by removing !important everywhere if it is to be avoided, but this leads to reduced robustness.

e.g. of the issue: