Best I can offer is a recommendation to change the 'Theme' to black - it
seems to offer the best 'clash' of colors in the title bar of the active
window, with the inactive window(s) being gray and the active window being
black.

To change the Theme, use the Office Button in upper left corner, then the
[Excel Options] button and select the Color Scheme you want in the 'Popular'
settings group.

Otherwise the best clues as to which is the active window is that there will
be the Excel icon and the minimize/restore/close buttons in the active window
title bar only.

"BimDim" wrote:

In Excel 2002, when there were two sheets in a window, we used to have a
bright border to show which was active and which was dormant. In Excel 2007,
the border colours are not transferred from Windows so active and dormant
sheets are all the same blue, silver or whatever. This is awkward to
handle.