chore: use dark logo always

[Oisin-M]

Description

The sidebar is always dark colour regardless of light/dark mode, so actually the logo for the sidebar should always be the dark mode one

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.