Studio: Fix spinner color when adding a site

[katinthehatsite]

After merging #362 , I noticed a small inconsistency in the design - the color of the spinner should be gray-70.

Before:

After:

Testing Instructions

• Pull the changes from this branch
• Start the app with nvm use && npm install && npm start
• Click on the Add site button to open the modal
• Click on Add site to add a site
• Confirm that you see the loading indicator in the sidebar with the correct color

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[matt-west]

Thanks @katinthehatsite!

[fluiddot]

LGTM 🎊 !