Your OpenWrt router can have different looks to the interface called themes. Typically the packages are called something like luci-theme-material
The default one is called "Bootstrap" with the menu items across the top. I installed two new ones, Material and OpenWrt2020 which have menus down the side. When the window gets narrow enough, either on desktop, but more typically on mobile, the menu gets compressed into a "hamburger" menu. I'm sure you've seen them before:
On Material, when the window gets narrow, it will look like:
This is what the menu looks like when opened like this:
To change the theme, you want to go to System, then System, then go to "Language and Style". I've taken screenshots from each of the different themes to show what it looks like to change the theme:
