[Cashtab] Added bg color to styled collapse

Summary: Added background color to styled collapse in StyledCollapse.js. Related to task T2174.

Test Plan:
cd web/cashtab
npm start
navigate to the configure tab
ensure background colors for both StyledCollapse components are correct and that content is legible

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10878