The WMF core features team tracks this bug on Mingle card https://mingle.corp.wikimedia.org/projects/flow/cards/403, but people from the community are welcome to contribute here and in Gerrit.
gerrit 103494 to update the mw-ui-button styles in core to latest Agora is nearly ready. The details of buttons are subtly different, the sizes are different. Before Flow can use this it has to harmonize its class names (I have a patch).
Flow's text input field styling (the blue bar on one side) is not yet in mediawiki.ui in core.