[Cashtab] Patch mnemonic input color OnBoarding.js

Summary: Related to T2603. Changed font color of mnemonic input description and lock icon in OnBoarding.js form.

Test Plan:
cd web/cashtab && npm start
open localhost:3000 in a private browser
observe that the lock icon in the form input is now blue
observe that the explanation text 'Valid mnemonic seed phrase required' is now white.

cd extension
npm run extension
in browser, navigate to chrome extension
if there are saved wallets in extension's local storage, delete them
observe that the lock icon, and the text ''Valid mnemonic seed phrase required'' are the correct color (cashtabBlue and white respectively).

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11852