If you pick a page on desktop (en.wikipedia.org) amboxes are totally missing. Minerva on mobile (en.m.wikipedia.org) is fine and displays the 'custom' styling that is implemented (I think) by MF.
The file applying .ambox { display: none } appears to be /resources/skins.minerva.base.styles/content/templates/ambox.less
For a page of interest, https://en.wikipedia.org/wiki/Alternate_history displays amboxes in #History of literature and #Cross-time stories, https://en.m.wikipedia.org/wiki/Alternate_history displays the expected MF styling, but neither is present in https://en.wikipedia.org/wiki/Alternate_history?useskin=minerva .