This should be pretty straightforward with Custom CSS and standard selectors.
As long as you are defining CSS for a specific form (Custom CSS defined as part of the form definition as opposed to Custom CSS defined via the plugin settings). Every form element has a unique ID which can be used as a CSS selector to select exactly one unique element.
For example, here is an element from one of my forms:
<div class="ss-form-entry">
<label class="ss-q-item-label" for="entry_1705367174"><div class="ss-q-title">What is the URL of the published form?
<label for="itemView.getDomIdToLabel()"></label>
<span class="ss-required-asterisk">*</span></div>
<div class="ss-q-help ss-secondary-text">This the URL Google provides when the form is published as a web page.</div></label>
<input name="entry.1705367174" value="" class="ss-q-short wpgform-required" id="entry_1705367174" type="text">
<div class="error-message"></div>
<div class="required-message">This is a required question</div>
</div>
So if I wanted to make the border around this input box 2px dashed red, I would do this:
#entry_1705367174 {
border: 2px dashed red;
}
Now what I think you want really is to apply CSS to a specific element which is a parent or sibling of this element which is a bit more complicated. Now unfortunately selecting the parent element of an element isn’t possible with CSS (maybe in CSS4) although it can be done with jQuery.
The best solution I know is to use the nth-child selector to specifically pick which element you want to apply CSS to. For example, if I wanted to apply CSS to only the second question on my form, I could use a nth-child selector like this to put a 2px dashed green border around the Div element which contains the input box, the label, etc.
div.ss-form-question:nth-child(2) {
border: 2px dashed green;
}
The option to add a class id to the containing DIV element does just that – it adds a class to the containing DIV so it can be used in custom CSS. It might be useful if you have two forms on one page and wanted to apply separate CSS to one versus the other. There are a couple of other use models as well.