I have tested it on my local environment, we cannot reproduce this issue, slicer works fine after published to Power BI services. In your scenario, could you please share your PBIX file, so that I can publish it to services and check if I can reproduce this issue or not.
I confirm to have the same problem. In Power BI Desktop is fine, selection on slicer is highlighted in black however in Power BI Web it is not the case, it is almost impossible to see any selection at all.
Re-publishing report did not help and the same is for all my reports within PowerBI Web.