Improve rendering of matplotlib image in dark mode

In dark mode, the text below figure is really close of the bottom of the figure. I think that we can change our css with a little trick:

```
/* sphinx-gallery */

/* add padding to images */
.bd-article img {
    height: unset !important;
    margin-top: 2%;
    margin-bottom: 2%;
}
```

We have something similar in scikit-learn by using `padding` but it is not super nice.