[web] fix secondary header shadow color

Summary: In the figma, I noticed that for secondary panel items (the panel items with a darker shade) the shadow color should have an opacity of 25% rather than 35%. This diff introduces that new variant and addresses that fix

Test Plan:
flow + please see the screenshot below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10563