For example you can modify the form's CSS if you go to your form > advanced >more options > form pieces > before form > add the following code in there :
// insert css code
Simply target the element you want to modify by targeting its ID or CLASS and add your custom CSS (right-click to browser->Inspect->and in Console find ID or Class).
(right-click to browser->Inspect->and in Console find ID for each element)
If you need more help, let me know and I'll help you.