luci-theme-material: CSS Fix for previous update

Commit b0f13efa

 Changed the color of a CSS element from a bright blue to
a dark blue. This resulted in a modal with difficult-to-read text (black
text on dark blue).

This commit sets the modal text-color to #000, in line with style
guidelines.

Signed-off-by: Quentin Baker <opensource@quentb.com>