Dark mode in the background of plugin-medium-zoom

## Feature request

Add dark mode in plugin-medium-zoom because it doesn't work natively

### Description

On the latest version of vuepress, you added the dark mode, but if I use medium-zoom on the dark mode, the background has been white yes

### Proposed Solution

Implement (and if already done, adding to the docs how to enable it!) the dark theme in the plugin
