Improve rendering of matplotlib image in dark mode

[glemaitre]

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.

[jeromedockes]

sounds good, why do we need the height: unset?