i´ve noticed that it looks much crisper here too and the text seems to appear a bit smaller. In the Cursor Tooltips dialog shown above, the two fields for changing the colors are grayed out here as well.
I was expecting that this could be changed from the registry:
HKEY_CURRENT_USER\Software\McNeel\Rhinoceros\5.0x64\Scheme: Default\Options\CursorToolTipSettings
but there are no keys for the colors only for the enabled state. 
however, when working with a display conduit using 2d text instead of using the built in ToolTips, it would be possible to draw a 2D rectangle behind the 2D text and control the colors as well.
c.