Update "reply" input container

What does this MR do and why?

• Remove gray background on reply field container
• Use white background for linked/internal note replies

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to issue
• Start thread and check reply field, default and expanded
• Create internal note thread
• Copy link to note

UX Paper Cuts 16.2 → Settings UI (#398215 - closed)