[Feature Request] Images for light and dark theme

**Describe the solution you'd like**
docfx has a switcher in the header that lets readers choose between the light and the dark theme. If applicable authors should provide both the light and the dark version of every screenshot used in the documentation.

**Additional context**
Add the `_dark.png` suffix to the name of the dark image version. This image will be then used when the user switches to the dark theme.

For example, save images as `example.png` and `example_dark.png`.