Action buttons not updating their color on Dark mode toggle

**Problem Description:**
The action buttons do not update their background colors when dark mode is toggled.
This is caused by the fact that the background color is in fact an image and thus it will not respond to the dynamic nature of UIColor's traitcollection-change callback.

**Steps to reproduce:**
Have a bulletin present on screen and toggle dark mode. The background color won't change to the correct value.

**Environment:**
- Device: Any
- OS: iOS 13+
- Version of BulletinBoard: Latest (as of 15 Oct., 2019)