This way it's easier to customize colors, particularly when links use
the same colors as the text surrounding them.
Note that the `markdown-editor-header` isn't displayed since commit
76b7f66fb, which was probably an unintended side-effect. So we're
modifying the colors here as well instead of removing the element; we
might display it again in the future.
Also note the change in `.supported` only affects the proposals section;
budget investments use a different color for the `.supported` sucess
message. Not sure whether this was originally intended or done by
accident.
335f0d8bde