Theme Horse

Hi frantisek.funda,

First of all create a child theme. Create style.css file and add the below code in your style.css file.

/*
Theme Name: Ambition Pro Child Theme
Author: Self-Help WordPress User
Template: ambition
*/

@import url("../ambition-pro/style.css");

body,
input,
textarea,
.main-navigation a,
h1, h2, h3, h4, h5, h6 {
	font-family: 'Lato', sans-serif;
}

Your child theme will be displayed. Activate your child theme.
Note: Please change above font family 'Lato', sans-serif to your own font-family.

Hope this may help you
Thank you!