I guess there is some conflict between plugins, we are trying to add CSS code but unluckily.
Maybe plugin don’t allow adjust CSS code for their form.
Can you contact them then ask how to custom CSS code for that form?
Check the screenshot below.
We are sorry, This feature is not available on our theme, So we can’t research to support you.
You should do practice more with this plugin.
It works fine on theme but you should adjust Style CSS for them. You can creat new LightBox template but you should adjust the style for form carefully.