You can create a file using a simple text editor for custom css and name it using this format: templatefolder-custom.css (e.g., rt_kirigami-custom.css or gantry-custom.css). For this issue add this to the custom CSS file#rt-header .gf-menu.l1 > li > .item {padding: 8px 15px;font-size: 0.9em;}
More Info here:
www.gantry-framework.org/documentation/j...custom_stylesheet.md