Update ColorThemeSwitch and its unit test

[TamiTakamiya]

Jira Issue: n/a

Description

As the pf-v6-theme-light CSS class is no longer used in PF6 chatbot, remove it from our ColorThemeSwitch class and update its unit test accordingly.

Testing

Steps to test

1. Pull down the PR
2. Run unit tests

Scenarios tested

A modification of a unit test case is provided in the PR. Also a manual test was executed using local chatbot instance.

Production deployment

• This code change is ready for production on its own
• This code change requires the following considerations before going to production:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
62.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud