Topbar redesign: replace brown background with beige

What does this MR do?

Replace the brown topbar background with a beige colorset.

How confident are you it won't break things if deployed?

it's just CSS so breaking (on a technical level) is unlikely. however:

• some people might not be used to this visually
• the contrast between page content and header might decrease -- especially when scrolling
• certain resources that were created with the old header color might need updating

so we should check for these and similar feedback points

Links to related issues

• Slack (german fs-design) https://yunity.slack.com/archives/C013ZTM7TUZ/p1605205704002100

How to test

Make sure to check different widths and logged in / logged out states :)

Screenshots

Before:

After:

Checklist

• visual change only, no test
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• no Beta Testing Repo issue needed

Release notes text

Wir haben die Menüleiste oben auf jeder Seite versuchsweise in ein helleres und freundlicheres Farbschema gebracht. Je nachdem, wie die Erfahrungen damit in den nächsten Wochen und Monaten ausfallen, sind weitere Änderungen oder auch eine Rückkehr zum dunkelbraunen Design denkbar. Unsere Designer:innen freuen sich über Input, falls ihr dabei mithelfen wollt!