Small python lab theming updates

[molly-moen]

A couple small theming things I missed in Python Lab:

• Theme the project picker modal
• Use the correct button color for the validate button

Screenshots

Validate button before (light mode)

Dark mode is unchanged

Validate button after (light mode)

Project Picker

Dark mode is unchanged.

Testing story

Tested locally.

Follow-up work

I'm still not sure what color the project switcher button should be in the workspace header. I started a slack thread and will follow up if we want to change it (right now it's black in light mode, which looks a little jarring).

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked