Set default background color

**Describe the bug**

Using version 1.13.0 on a dark background looks like this:

Is this behavior intended? I mean, I've fixed this via css, but maybe there should be a config option? I am not sure what is best to use, so I just want to ask here.

This is my scss fix:
```scss
.EasyMDEContainer {
background-color: white;
border-radius: 4px;
}
```

No source file, test, or entry point is named, and the report asks whether a CSS fix should become a configuration option. Begin by locating the EasyMDE container styling and configuration paths; done requires an agreed default or option and a check for the dark-background case.

Written by the indexing model from the issue text.