Make the multi select buttons change colors when selected in drill creation

Add functionality to change the color of multi-select buttons when selected in drill creation.

CSS Changes:
- Add CSS rules in src/routes/styles.css to change the background color of multi-select buttons when selected using the .selected class.
Svelte Component Changes:
- Modify src/routes/drills/create/+page.svelte to apply the .selected class to multi-select buttons for skill levels and positions when selected.
- Add reactive statements to update the class of the buttons based on the selected state.
Cypress Test Changes:
- Update cypress/e2e/drill_creation.cy.js to include tests verifying the color change of multi-select buttons when selected.
- Use should('have.class', 'selected') to check if the button has the .selected class.

For more details, open the Copilot Workspace session.

austeane / qdrill