Closes#930.
We already had classes for this kind of feature, but it was used
specifically for repository information. Just renamed the classes so it
is clear how they can be used.
To use them with markdown, just do it like this:
```markdown
{: .only-light}
{: .only-dark}
```
and with html:
```html
<img
class="only-light"
src="assets/img/prof_pic_color.png"
>
<img
class="only-dark"
src="assets/img/prof_pic.jpg"
>
```
Signed-off-by: George Araújo <george.gcac@gmail.com>
a9da28e256