Form elements are defined in base/form.css can be styled by adding form-input to your desired element.
:root {
--input-radius: 0.25em;
--input-border: 1px solid var(--color-gray-light);
--input-border-hover: 1px solid var(--color-gray);
--input-border-active: 1px solid var(--color-primary);
--input-shadow: 0 1px 1px #0000000f;
--input-shadow-hover: none;
--input-shadow-active: 0px 0px 0px 3px var(--color-primary-a-10), 0px 1px 4px 1px var(--color-primary-a-20);
--input-check-radio-size: 18px;
}