### Steps to reproduce

* Access OpenProject in Dark mode


### What is the buggy behavior?

* The sidebar background colour is `var(--main-menu-bg-color)` that's darker than the rest of the application

* This var is mapped to the dark overlay background colour

* Inconsistent with Light mode, where the background colours of sidebar and main application are the same


<figure class="image op-uc-figure"><div class="op-uc-figure--content"><img class="op-uc-image" src="/api/v3/attachments/777150/content"></div></figure>

### What is the expected behavior?

* Change sidebar background colour to `var(--body-background)`

* \[open\] Change top navigation bar background colour to the equivalent of Begin of the insertion`--bgColor-inset`End of the insertion Begin of the deletion `--overlay-bgColor`End of the deletion

* Also for consistency with Light mode, where the top nav bar is a different (shade). This offers more hierarchy and structure to the page.


**Expected colours:**

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/777153/content">

### Environment information

**Community**
**OpenProject 16.4.0**

Core build: [c3a37508b5b55d170e02f5a310cf3e073e1d2d39](https://github.com/opf/openproject/commits/c3a37508b5b55d170e02f5a310cf3e073e1d2d39)
Product version: [c178fd38ad67eaeae199ca1ffbb7c449d637bd79](https://github.com/opf/saas-openproject/commits/c178fd38ad67eaeae199ca1ffbb7c449d637bd79)
Core version

**Browser:**
Firefox Mac

**Language**
EN