For Bootstrap 2 / Real estate agency portal

Default font can be changed with css file customization, templates\bootstrap2-responsive\assets\css\styles.css, line around 257 or find code and change:

font-family: "Open Sans","Arial",sans-serif;

For Bootstrap 4 / Real Estate Geo Portal

Default css files for template:

\templates\bootstrap4\assets\css\winter-flat.css

\templates\bootstrap4\assets\css\custom.css

For change font size of menu, add into \templates\bootstrap4\assets\css\custom.css

.menu .nav-link {
    font-size: 12px;
}

Similar way also for other themes