The current recolorization code only recolors paths and rectangles. In
particular it leaves circles and ellipses as-is.
I opened the merge request gtk!214 for this issue. But in the meantime,
it is quite easy to change circles and ellipses into path with "object
to path" in Inkscape.

A second problem is that it only recolors fills, not strokes, as
previously stated. So I need to remove strokes from icons, or transform
them to paths.