Make the running logo "arsnova.click" compatible with the current theme

The four colors of the logo are too dominant and sometimes quite anoying. They were chosen for the very first theme and don't harmonize with the basic colors of the later themes.

I suggest to define two Sass colors:

• $logo_color for all individual parts of the logo: "ars", "nova", "dot", and "click". This color should be preset to white.
• $logo_running_color which should always be the background color of the hashtag input field, i.e. $input-background-color.

Thus, every theme will focus upon the "running" URL of the app and on what to do first, that is to insert the hashtag.