Works, decent structure... less opinionated, similar to bootstrap, bulma or any number of other CSS libraries.
Kind of would like to see one of these css library/frameworks much more opinionated. For example, just make the baseline button as well as submit/reset form types the default button composition (no btn class needed). Header elements should also be styled by default. Similar for stylized radio/checkbox items...
<label><input type="radio" .../><span>text</span></label>
As a convention is enough to custom style in modern browser with the nearest neighbor, the label wrapping the input will cause the label to work without the need of a "for" attribute.
The class soup just gets a bit weary after a while, and largely unnecessary.