There are two general colour settings for plain text: background and text. While text sets the colour for the text background sets the colour for the background right behind the text.
Both settings apply to unrendered Markdown too because unrendered Markdown is plain text.
The same goes for the background and text settings in full screen view.
But full screen has another setting, Width. For plain text and unrendered Markdown this setting sets, as should be expected, the line width of the text.
So it does for rendered Markdown. But in rendered Markdown the Background colour setting does not affect the colour right behind the text. This becomes visible if Width is set to less than 100 %—which is highly recommended at least on larger screens—and if the rendered Markdown background colour is different to the colour setting in Background for full screen. Then there is a box, let’s call it “the paper”, i. e. an area from the top to the bottom of the screen that has the width defined in Width.
If the user has not defined a background colour in their Markdown CSS it is white. If, say, the background for full screen is set to black the result is white “paper” in the middle of a black screen. Which is very pleasing to the eyes.
But it has a downside: It only works for rendered Markdown. When switching from rendered to unrendered Markdown the settings black backround plus black text result in a completely black screen.
So it would be nice if there was a distinction between background and backdrop (or whatever terms are more apt) for all kinds of content. Background for the colour of the box defined in Width and backdrop for the colour outside of that box.
In my opinion this would increase the usefulness of Full Screen Mode immensely.
And it would be nice if the Next/Previous Document and other Go keyboard short would work in Full Screen Mode too. (By the way: Although the keyboard shortcuts are shown correctly in the Go menu cmd-Arrow down does not move one document down but to the end of the document list while cmd-page down moves one document down. Same with up.)