Three choices. If you just want this inside the editor, and leave any export styling to the exporter itself, you can just overwrite the background color of the mark element in the editor.
If you want the background color to be matched both in the editor and your exports, but prefer to keep everything in your file, you won't be able to use the mark syntax itself, but you can use Pandoc spans: [highlighted]{background-color=green} (the syntax may be off, but this is the general idea)
Lastly, if you want full control, you'd want to at the same time overwrite the background color of the mark element within Zettlr and then also adjust how the various exporters style the highlights, which depends on how the exporters work.
