The best way is to override the existing styling for the H tags (h1, h2, h3, etc) or if there is on in particular such as H3…the styling for the headings is found in the style.css around line 232
h1, h2, h3, h4, h5, h6{
font-family: alex brush,open_sans, sans-serif;
font-weight:400;
color:#333;
margin-bottom:16px;
margin-bottom:1rem;
}
However, if you want the styling you your page like you pasted, your code is actually incorrect. To add (what is called inline styling) a style to an element, you would do it like this:
Weddings
If you are using a plugin for Google fonts, then you would just do the H3 selector only with the font you want, size, etc.