Fix expiry date in theme set-cookie

[timhawes]

Description:

The theme cookie has an invalid expires date:

Set-Cookie:  theme=light; expires=time.struct_time(tm_year=2024, tm_mon=10, tm_mday=19, tm_hour=9, tm_min=22, tm_sec=51, tm_wday=5, tm_yday=293, tm_isdst=0); Path=/

With fix:

Set-Cookie:  theme=light; expires=Sat, 19 Oct 2024 09:22:58 GMT; Max-Age=31536000; Path=/

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• Tests added

[caronc]

🙏 Thank you!