To enable this all you need to do is set the colour:
- click [Settings] and select 'Style',
- click the 'Inactive window overlay' swatch
- in the 'Select a Colour' window, optionally pick a colour
- set the Alpha value to, say, 64.
- click the tick button
You can save the colour variation as a new preset (or overwrite your existing one):
-- click the 'Presets' button in the 'Colour settings' panel
-- click [Add] on the left-hand side to add a new folder, named 'iwo', perhaps,
-- click [Add] on the right-hand side to add the colour combo.
or
-- click the 'Presets' button in the 'Colour settings' panel
-- browse to your currently-loaded preset
-- select it,
-- click [Replace] on the right-hand side to replace the colour combo.
Here are a few examples (using a highlight colour, black and blue respectively). I think I prefer to use the same colour as the 'Highlights' colour (that may be because my preferred Styles use a grey background).
... ...
Big pics:
http://i.imgur.com/JE7zEHK.png
http://i.imgur.com/QJquDH8.png
http://i.imgur.com/lou2qgA.png
If you have a fond feeling for the Windows "Stopped responding" appearance, select White and an Alpha value of about 160.