Rofi is a replacement for dmenu on i3 and other window managers. It allows you to have a visually more appealing launcher with more features than dmenu. Rofi has themes and a theme editor to allow you to alter the way it looks.
Installing rofi is very easy on any system.
FreeBSD
# package install rofi
Linux Systems
# apt install rofi
or
# dnf install rofi
One nice feature with rofi is when you install it you get the rofi-theme-selector. Once you run it you can choose from many different themes for rofi. I use Adapta, which is kind of a tan muted color scheme. There are 32 themes to choose from. One is bound to blend in with your color scheme on your desktop and tickle your fancy. Or you can go rouge and create your own. Below is a camera pic of my rofi. Could not get a pure screenshot of it as rofi disappears any time you hit another button or click anywhere. Same with the theme selector.
There are three different tabs across the bottom, window for window switcher, drun for programs that run from a menu, and run for running command line commands. I usually dont use anything but the drun part. I use the keyboard shortcuts or mouse for window switching, and use a terminal for running commands.
Now for i3wm, you go comment out the line that has dmenu in it and uncomment the line that has rofi listed in it. Simple as that to switch over after you install it. Other window managers can use rofi, but I have never set it up on any of them, so google is your friend for that.