Show screenshots matching the theme #71

For some screenshots, it might be better to use the version matching the current theme (dark mode on/off).

This should be possible using dark:hidden and dark:block variants:

<div class="block dark:hidden">

![alt](./light.png)

</div>
<div class="hidden dark:block">

![alt](./dark.png)

</div>

Wrapping in <div> because we can't use <img> tag with relative paths in Markdown at the moment.