Hi quat, in MoI version 4 that border comes from a bitmap image file, it's the file ViewportForeground.png inside the ui subfolder in MoI's install. So to change the color edit that PNG bitmap image in an image editor.

In the MoI v5 beta there is a new theme editor and it's generated from the settings in the theme editor in the Viewport section, under Border color, Border radius, and Border thickness.

- Michael