Window Controls Icons don't get color indication when focused with keyboard

[knoobie]

Currently with the default theme Valo we don't get a color indication when the user uses a keyboard to navigate between the controls of the window. This can be quite depressing when you have to tab like 3-4 times just to find the current focus on the page after a window opens.

Vaadin Version
7.3-8

Example
http://demo.vaadin.com/sampler/#ui/structure/window

Steps to reproduce:

• Open the Example URL
• Press "Open a new Window"
• Focus the Window
• Use Tab to navigate between the Window Controls or use a developer tool to imitate focus

I think it could be easily fixed here https://github.com/vaadin/framework/blob/8.0/themes/src/main/themes/VAADIN/themes/valo/components/_window.scss#L178