Draft: Add a custom stylesheet for better distinction between images and text

In most documentation pages, the screenshots have a white background, which makes them uneasy to distinguish from the text.

Add a CSS rule for all screenshots in order to add a margin on the left, and a thin black border around.

There is currently no custom CSS file, so create one in a new "styles" directory.

For all figures with width="100%", reduce them to "90%", so that we have the same margin (5%) on left and right sides.