Problems with editing CSS

Hi guys,

I have some problems with editing CSS in the contact form that I have made on my website footer. I want to make different font size and font weight but the CSS code that I enter does not give any result. I have added the following entry to my “Additional CSS” section in Hestia Child Theme:

div.wpforms-container-full .wpforms-form .wpforms-field-label {
    display: block;
    font-weight: 300;
    font-size: 14px;
    float: none;
    line-height: 1.3;
    margin: 0 0 4px 0;
    padding: 0;
}

but it does not give my any result and as I have noticed, it comes back to the default CSS when I try to inspect that.

Can you give me the reason why is that happening and why I cannot customize it?

The page I need help with: [log in to see the link]