[Cashtab] Signature Copied text color patch

Summary: Rekated to task T2434. Patched font color of "Signature copied to clipboard" text in Send.js to lightGrey.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
expand the Sign Message collapse and generate a signature
click on the 'Signature:' text area
observe that the text "Signature copied to clipboard" is lightgrey

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11409