Clean up stylesheets

[oyvindrobertsen]

Colors, fonts and other things that are nice to have defined in one place should be moved to a constants.scss.

[oyvindrobertsen]

We should use this issue to discuss further what changes we want to do on a structural level to stylesheets.
Some thoughts:

1. We are currently using only small subsets of Bootstrap. We should look into migrating to something more lightweight.
2. It would be nice to have a clearly defined color-palette/style guide.