customizable look and feel for portal

[baldarn]

Comprehensive Summary of your change

This PR aims to let a user configure the look and feel from the harbor.yml file

Issue being fixed

Fixes https://github.com/goharbor/harbor/issues/20847

Please indicate you've done the following:

• [ ] Well Written Title and Summary of the PR
• [ ] Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• [x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [ ] Made sure tests are passing and test coverage is added if needed.
• [ ] Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.