Selecting buttons and checkboxes gives no visual feedback

1. Go to https://archive.softwareheritage.org/save/
2. press tab a few times; this cycles through items in the sidebar without highlighting them (though it is somewhat visible because they are links and browsers announce their target)
3. when you reach the "origin url" field, press "tab" one more time
4. This selects the "Submit" button, but its style is completely unchanged.

Same issue on https://archive.softwareheritage.org/browse/search/ when tabbing through checkboxes

By the way, the contrast of the search button (both on https://archive.softwareheritage.org/browse/search/ and in the header of other pages) does change when selected, but isn't very visible.

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Migrated from T4060 (view on Phabricator)