This mockup makes the following changes to the current action icons design:

1. All icons are Material Design (to improve responsiveness)
2. References icon is a paper
3. Unhelpful is now a downward arrow
4. Helpful is now an upward arrow
5. Bookmark is now filled to match the rest of the icons
6. Mastered is now an eye
7. Child nodes is now a back and forth arrow
8. "idea" node icon is now an illuminated icon
9. Node height icon is now a single double-headed arrow
10. Number indicators for icons are lowered

All icons are 18px in size, following Material Design guidelines.