Outline font (glow effect) in the sheet tabs
Excel automatically switches the colour of the sheet tab font to either black or white to provide high contrast with the tab colour for unselected sheets. However, there is sometimes a grey area where neither black nor white is easily readable (e.g. the "green" standard colour).
In Excel 2016, the tab colour is lightened for selected sheets and the font colour is always green. For dark tab colours (e.g. black), the sheet name can be hard to read.
My suggestion is to keep the sheet tab font white and outline it in black (or possibly let the user choose the colours). Desktop icons clearly demonstrate that outlined fonts are easily readable against any background.