[Regression] Inconsistent dark theme background

[YuHsuan-Hwang]

Describe the bug

There are some changes to the dark theme background in this dev cycle (possibly results from upgrading to bp v5):

1. The menu buttons have a darker color comparing to the overall background.
   

   cf. v4.1: Consistent background.
   

2. Among all the widgets, z profile, histogram, and spectral line query widgets has a lighter color comparing to other widgets.

   

   cf. v4.1: All widgets have the same color.

3. Undocked widgets have a lighter color comparing to docked widgets. This includes all undocked normal widgets, all settings widgets that cannot be docked, and PV previewer and catalog scatter/histogram plots. (Dialogs have the same color with the docked widgets.)

   

   When we export png file of the profile, we always get the light background (undocked widget background) instead of the dark one (docked widget background).

   cf. v4.1: Docked widgets, undocked widgets, and dialogs all have the same color.

Expected behavior
Consistent background color.

Platform info (please complete the following information):

• OS [e.g. macOS Monterey]: macOS 14
• Browser [e.g. chrome, safari, electron app]: chrome
• Browser version [e.g. 22]: 129
• Backend branch [e.g. dev, v3b2 release]: dev
• Frontend branch [e.g. dev, v3b2 release]: dev