Invert light images in dark themes

[avivace]

CSS filter: invert(1.0) can be easily applied to almost every image when a dark theme is selected. The result is very nice and will allow us to not rewrite/redesign every image for the dark themes.

However, we need a "not-invert" class for the images for which this effect doesn't work very well or the "themed" SVG already pointing to the CSS variables of the selected theme.

[ISSOtm]

The "themed" SVGs don't use the <img> tag, so they wouldn't be affected anyway.