Check out this codepen and let me know if this is what you are wanting to do.
If this looks correct, you can spy the css in the css panel of that pen. I made a comment so you can see what code is relevant. You will probably want to lay it out 1 field per row on mobile so I set the min width for it to be 500px. Anything under that will be on one line.
Also, if you are embedding this form on a site that is not on hubspot, you will need to set the form to "raw html" (see screengrab below). The consequence of this is that it will strip all of the default styles from the form.
If you need some help with anything else here, please reach out! I hope this gets you going in the correct direction!