[Cashtab] Add bg color ant select-selector

Summary: Related to task T2177, bg color needs to be added to ant-select-selector in EnhancedInputs.js.

Test Plan:
cd web/cashtab
npm start
ensure that drop down menus in Send.js and Configure.js have an appropriate bg color and that content is legible

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10845