[web] update ModalOverlay component

Summary:
updated ModalOverlay component by changing background-color of overlay to hex value because hex values are more frequently used in the codebase, and changed useEffect to useLayoutEffect. Reasoning for that here

Depends on D5363
Linear Task: ENG-2038

Test Plan:
Please watch the video to see that there are no visual or performance regressions with these updates to the ModalOverlay component and everything works as it did previously

Reviewers: atul, rohan, abosh, przemek

Reviewed By: atul, przemek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5364