TextField, NumberField, Dropdown, SearchableDropdown, and Toggle all
painted their background, border color, and border width with the same
three-line ternary ladder (`_focused ? focusFill : _hot ? hoverFill :
normalFill`). Five components × three properties × three branches is a
lot of room for one of them to drift from the others when a new state
ever gets added.
Add controlFill / controlBorder / controlBorderWidth on Style and rewrite
each call site as a single binding. No visual change.