@yinglin1202: To increase the height of the menu bar, you need to increase the line-height in the following CSS and then add it in “Appearance => Customize => Theme Options => Custom CSS” box.
.catchresponsive-nav-menu a {
line-height: 46px;
}
About the font change, either you need to upgrade to pro and you will have font family options where you can choose from popular google fonts and web safe fonts. If you want to stick with Free version, then you either need to use font plugin or build child theme and add google fonts.
Font css will be as below:
.catchresponsive-nav-menu {
font-family: "Open Sans",sans-serif;
}