Section line should be bolder than grids header line
Make section line thicker by 1px. Make extra thickness of table header row line (underline) thinner by 1px
Indeed, this does not look very good, I'm leaning towards your proposition, it seems cleaner.
after checking this I would propose some different variant, as IMO this would look very bad.
So couple of other proposals. I think that colorful section lines add too much noise. My favourite is with section separator made 1 px thicker (2px) and table header line 1 px thinner. WDYT?