Unify CSS to ease maintenance

[noqqe]

Currently CSS is used differently on pages. There are some pages (especially forms) that use own classes or worse use ids. The goal of site wide CSS should be to have a common look and feel. It would therefore be better to define a common set of CSS classes (i.e. for form fields, buttons, ...) and use these throughout the site.

Using element ids in CSS selectors has to be a last resort in case of trouble using CSS classes or to restrict CSS to specific elements on the site.