paper-styles is missing css variables for icons in default-theme.
default-theme.html does say --dark-theme-secondary-color: #bcbcbc; /* for secondary text and icons */, however that is incorrect, dark theme icons don't match the secondary color.
Here's a summary of the opacities for light and dark theme text and icons:
| Text / Icon |
Light Text |
Light Icon |
Dark Text |
Dark Icon |
| Secondary / Active |
54% |
54% |
70% |
100% |
| Disabled / Inactive |
38% |
38% |
50% |
50% |