https://mlich.zam.slu.cz/js-matter/js-bridge/bb-edit.htm
#levels button {background: linear-gradient(0deg, transparent 0%, #8e8583 0%, #8e8583 31%, transparent 31%); border: 2px solid #333; padding: 0; margin: 0 3px 6px 0; height: 135px; } /*#b6d9f0*/
In link, you can see on some buttons top white line, on some not. I trying add transparent before gradient, sometime works, sometime not 
When i set only background: transparent, buttons have no top line.
I thing, minimal code must work as background: linear-gradient(0deg, #8e8583 31%, transparent 31%); work, but with more lines