[Cashtab] Airdrop modal percentage color patch

Summary: Related to task T2436. Loading percentage in Airdrop modal text should be lightGrey.

Test Plan:
cd web/cashtab && npm start
navigate to airdrop screen and paste eToken id
or
navigate to eToken information screen and click 'airdrop xec to holders'
input an amount of XEC to be calculated
observe that the percent text near the progress bar is lightGrey instead of black

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11410