adjust header styling & drop overview screenhot

[HenrikeW]

Related issue(s) and PR(s)

This PR closes #478

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

List of changes made

• the nav bar in the header got a new font color, the elements got a new border radius
• the overview screenshot on the help page is dropped because it's hard to keep it up to date and it's not super necessary

Screenshot of the fix

Testing

• visit the report page & the help page and see how it looks

Further comments

Definition of Done checklist

• I have made an effort making the commit history understandable
• I have performed a self-review of my own code and commented any hard-to-understand areas
• Tests and lint/format validations are passing
• My changes generate no new warnings