[Cashtab] Patch character limit counter font color

Summary: Related to task T2421. In Advanced collapses that contain a character counter, the counter font should be lightGrey, not black.

Test Plan:
cd web/cashtab && npm start
navigate to Send page
expand all three of the collapses
observe that the character counter at the bottom right of each collapse is lightGrey, not black

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11387