css style for forms

Hi, Great theme by the way. Ive recently downloaded an add in called pta sign up which is the functionality need for my site. The add in contains no css so takes the css from the theme. i am having 2 issues, the form that displays is poorly formatted and i seem to be unable to target it, everything i do to try and change the form (pta_sus_signup_form) doesn’t work. i just need to line up the text with the actual input box…please help. https://www.friendsofysgolpenybryn.org.uk/volunteer-here

also when viewing this add in on phones the formatting is crazy.

ive been in touch with the developer who made the add in and he explained that there is no css so it works better with the installed theme.

any help or guidance would be appreciated

Many thanks