Unify modal button styles with ThemedButton

[chriswilty]

While we intend to rework the styling for modals (see #578 and related issues) it would be nice in the meantime to unify the styling of their buttons to match the action buttons used elsewhere in the application (e.g. header bar actions, defence reset buttons).

Same colours as action buttons:

Default

Focused

Hover

Focused + Hover

Same colours as level buttons (according to new styles in #777):

Selected, with or without hover

Selected + Focused