Cool! very nice. I like this form, the design is kind of good, like the background image. But it needs some fix.
First the good parts
placeholder for text fields and textarea, very good.
all radio and checkbox buttons come with associated label, very good.
there is a non selectable disabled option for combobox, but it’s not selected by default, please fix.
No global font-size overriding, very good.
Now issues
Beside the page is responsive, but in mobile view, there is a little too much margin/padding from screen edge, so it wastes some notable spaces.
I also suggest text fields nad combobox go to fit with form width in mobile view.
The fix is very easy as I checked the layout coding. first considering browser applies default margin/padding of 8px(or 0 or less, or more) by default for body. So for mobile you may override this value by margin:0 for body.
Same your main form goes for 80% and 90%(for mobile) which could work for 100% too, try it.
I think the transparent background colour of form is a little too much transparent, I suggest you bring moe solidness(less transparency) to it.
I also think placing the submit button inside the coloured form looks better, but that’s my opinion. Also try to make it more sharper.
You may also check my survey form challenge walkthrough article which I stated these notes there too with samples and explanations(feedback is appreciated too)
Like to see some update and great progress soon, keep goin on great work, happy programming.