[FLORA-418] Fixed text color for header and button in login page

[mau5mat]

Proposed changes

I have worked on the task detailed at #418 , please see the attached images below compared to what was shown in the task and merge if you think my fix is satisfactory.

I have also updated the CHANGELOG.md to reflect the change and link to the pull request.

This is my first pull request for an open source project and I have tried to keep to the guidelines provided as best as I could, I apologize if I have made any errors!

Contributor checklist

• [x] My PR is related to https://github.com/flora-pm/flora-server/issues/418
• [x] I have read and understood the CONTRIBUTING guide
• [x] I have inserted my change and a link to this PR in the CHANGELOG

[tchoutri]

@mau5mat fantastic! I'll let you merge it. Don't mind the CI failure for the nix build.