Change Visual Studio 2013 Summary Tag Color on Dark Theme

have a look at:

Tools => Options => Environment => Fonts and Colors => Text Editor => XML Doc Tag

Hope it helps :)

The above answer didn't work for me with VS 2015.

But this did the trick:
Tools => Options => Environment => Fonts and Colors => Text Editor => XML Doc Comment - Name
Change the foreground color.
Note: This changes the color of 'summary' tag only. You can play around with other XML Doc Comment items to suit you preferences.

In Visual Studio 2015, the following will change the slashes, summary tag, and description:

XML Doc Comments - Delimiter

XML Doc Comments - Name

XML Doc Comments - Text