When running in Mac OS Mojave’s dark mode, clicking in Excel’s formula bar to edit other than a simple formula or function, some of the colors Excel assigns to the different components of the formula are too dark to see the formula component against the dark formula bar background. This makes it impossible to see those parts of the formula for editing purposes. (See attached image.)
I haven't found any setting either in the Mac OS or in Excel to modify the colors Excel assigns to formula parts. The only solution I've found is to switch off the dark mode when using Excel. But, I prefer the dark mode, so earlier today I sent this issue to Microsoft using the "Send a Smile" icon in the top-right corner of Excel, but I'm wondering if other Mac users of Excel have noticed this, too.
Thanks, alangston, for the suggestion to change the default accent color in Mojave from graphite to white to fix the problem. Unfortunately, though, changing the accent color is system-wide on the Mac, which results in white highlighting being lost on white backgrounds wherever else this feature is used.
I think the real answer is for Microsoft to correct this problem within Excel.