Add Custom Theme option

Related issues

Addresses #123 (closed)

Acceptance criteria

• Implemented code changes don't diverge from the related issue.
• Documentation reflects the status quo after merge.
• Version number is incremented according to SemVer. (Snapshot versions are not merged/released.)
• Only released versions of dependencies are used (i.e. no snapshot versions).
• Pipeline still works.
• GitLab issue is up-to-date.

If necessary, the following sections have been updated:

• Swagger
• Readme.md
• Documentation

Proposed squash commit message

Add Custom Theme Option

- Add import to style.scss
- Add example to _se-custom.theme.scss
- Add logo class to title bar
- Add custom otpion to README