Update checkbox styles for Fluent theme

All disabled checkboxes should use the gray 400 instead of 300

Аlso the indeterminate state of fluent theme should be updated to this shape:
<img width="83" alt="Screenshot 2022-03-10 at 12 14 07" src="https://user-images.githubusercontent.com/1702064/157640477-02762566-e814-482f-9a34-84ec0077cf97.png">