replace pure css icons with svg data icons (base theme)

[raffaelj]

I really like the pure css icons, but they are affected by rounding issues in different browsers. Using SVG data uris instead keeps the benefit from using a single (css) request to have all icons, but it might be harder to style them.

see:

• https://css-tricks.com/probably-dont-base64-svg/
• https://codepen.io/tigt/post/optimizing-svgs-in-data-uris
• https://codepen.io/jakob-e/pen/doMoML