How Do I Get Custom CSS to Work?

I Downloaded the plugin “SiteOrigin CSS”, And I noticed that the Visual and Expand Features are not working properly for me… I also noticed at the bottom right before the “Save” button, it says “Changes apply to OceanWP and its child themes”….
I don’t understand what that means?

I am trying to Customize my Signup form to this CSS Style:

.signupform {
margin: 0 auto !important;
max-width: 460px;
padding: 30px 0;

.signupform > p {
float: left;
margin-right: 10px;
max-width: 240px;

.pum-content.pop-content {
background: #fff none repeat scroll 0 0;
border: 3px dashed red;
padding: 30px;

.pum-content.pop-content > p {
font-size: 24px;
margin-bottom: 0;

.signupform .wpcf7-form-control.wpcf7-text.wpcf7-email {
background: #fff none repeat scroll 0 0 !important;
border: 1px solid !important;

.signupform .wpcf7-form-control.wpcf7-submit {
background: #77AA05 none repeat scroll 0 0 !important;
color: white;

.wpcf7-response-output.wpcf7-display-none.wpcf7-validation-errors {
border: medium none;
color: green;
display: block !important;

But I am having no luck.. If you want to check out my page, it is:
I do have Elementor, Ocean Extra, Pop-up Maker, Contact Form 7, and All-in-one-video Gallery plugins… Not sure if these plugins are causing this error to happen? Please help?

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