fix(NotificationsPanel): remove hard-coded dark theme

[paul-balchin-ibm]

Closes #5259

NotificationsPanel now inherits from parent theme.

What did you change?

packages/ibm-products-styles/src/components/NotificationsPanel/_notifications-panel.scss

How did you test and verify your work?

• Storybook

Themed output

The first notification in each screenshot shows the hover state when the mouse is over the close ("X") button.

white	g10	g90	g100

Empty state

white	g10	g90	g100

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.51%. Comparing base (e26d6dc) to head (4ac9998).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6862   +/-   ##
=======================================
  Coverage   81.51%   81.51%           
=======================================
  Files         399      399           
  Lines       12975    12976    +1     
  Branches     4268     4269    +1     
=======================================
+ Hits        10576    10578    +2     
+ Misses       2399     2398    -1     

Components	Coverage Δ
ibm-products	∅ <ø> (∅)
ibm-products-web-components	∅ <ø> (∅)

[elycheea]

Noticed some small things when I was posting to the team channel for design review —

1. Missing borders on the panel (compare to the switcher). Probably also should still have a shadow since it’s a floating element?
2. Empty state theme! Looks like the theme is hardcoded here — might need to do a some theme detection so that it switches between light/dark since empty state component used here relies on the theme prop.

[paul-balchin-ibm]

Great catches: added borders, drop shadow, and a themed empty state illustration.