Hi Bombata,
You can easily add your own stylesheet by adding <link type="text/css" href="<<link to stylesheet>>" rel="stylesheet" /> to a literal datalabel, you can do so in the "When the server loads a View" event to avoid style jumps. If you have no way to host a CSS file somewhere publicly accessible, you can always inject the styles using the <style>...</style> tag.
Note that the selectors will be smartforms-specific (so you can't just load an existing style like bootstrap or material design and expect it to work). If you want to use existing stylesheets, you can use JavaScript inject to add the necessary classnames/identifiers to controls to facilitate the necessary selectors.
Cheers,
Ernie