It's in the theme somewhere. In your attached image the highlighted text (Box) is Cyan (pale blue - 133,255,229). I just checked mine which is the default out-of-the-box theme and the highlighted text is white (255,255,255). The .clr file contains a LONG list of colours, but nothing to relate them to the on-screen controls which use them. The only way I can see to find this is to alter every colour in the list one by one checking each time if you've found it.. This could be a long job...
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).