###

### Steps to reproduce

1. Login and go to Time tracking module in dark mode


### What is the buggy behavior?

* Insufficient color contrast between the graph and the background


<img class="image_resized op-uc-image op-uc-image_inline" style="width:503px;" src="/api/v3/attachments/831904/content">

### What is the expected behavior?

1. Minimum contrast ratio of 3:1 Begin of the insertion

2. Add a border with borderColorDefault in both light and dark for the empty bars.End of the insertion


###

### Environment information

**OpenProject version**

_2.0.0 (585)_

**Operating System**

* [ ] Windows
* [ ] Mac OS X
* [x] Mobile iOS
* [x] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)