fix(core/menu): fix theme switching icon

[matthiashader]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Currently the icon for the theme switching is the light bulb, which is normally used for idea or insights.

GitHub Issue Number: N/A

What is the new behavior?

The icon has changed to "light-dark" in order to make it more clear.

Does this introduce a breaking change?

• Yes
• No

Testing

Other information

[danielleroux]

Please add a visual regression test.