Add theme variables for icon colors to default-theme

[dantman]

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%