[Cashtab] Patch font color for One To Many Airdrop Payment Outputs input

Summary: Related to T2692. Fixed font color of One To Many Airdrop Payment Outputs input in Airdrop.js by changing CSS in the AntFormCss styled component in EnhancedInputs.js.

Test Plan:
cd web/cashtab && npm start
Click on eTokens from the wallet page
Select a token
Select Airdrop XEC to holders
Input an amount in the Total XEC airdrop input
Click Calculate Airdrop
Observe that the text 'One To Many Airdrop Payment Outputs' over the large input is now white.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12079