Right click + save > to .../Program Files/Construct2/themes
Also ignore the fonts, as themes do not override them.
(fonts in screenshots are proFont, and Calibri)
MonoDark
<img src="https://dl.dropboxusercontent.com/u/53374990/Pics/darkTheme.png" border="0">
MonoDarkOutline
<img src="https://dl.dropboxusercontent.com/u/53374990/Pics/DarkThemeOutlined.png" border="0">
GcBdark
<img src="https://dl.dropboxusercontent.com/u/53374990/Pics/darkGcB.png" border="0">