[Cashtab] Add css prop to TxLink in TxHistory.js

Summary: Added CSS props to the TxLink styled component in the TxHistory component in order to further progress on dark theme. Related to task T2171.

Test Plan:
cd web/cashtab
npm start
Ensure that the text color in the TxHistory view is legible.
In IDE:
Change color reference in theme.js
Back in browser, ensure that color change has taken place.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D10838