In Data > View this looks fine. Go to Base > Single mean (which has a filter) and then back to Data > View and you will see the css has been changed. This happens for any page with a slider (e.g, all tools in Base except Cross-tabs, Regression (OLS and GLM, but not Correlation which doesn't have a slider). To test this some more I added a slider to the Data > Visualize tab and the same effect occurred. Switching back and forth changes the CSS.
Note that this effect also occurs when I comment out the CSS listed above. In that case the default CSS for the column headers/footers is still changed when switching back and forth to pages with shiny sliders.
Can you provide two screenshots that show how the styles change from one case to the other? I could see some subtle padding differences, and that does not seem to be too bad in my eyes. Perhaps you meant something more significant that I failed to see.
Just an FYI ... without the clear button the text from different cells can overlap. See screenshot. Like I said, not a big deal but enough for me to prefer plain = TRUE for now. Thanks for that option!
Update: Just to be clear ... the overlap only occurs after visiting a page with a shiny slider in it.