Alternate themes can be easily installed with the RetroPie Theme Installer.

RetroPie Setup Script >> Setup >> EmulationStation Themes

Access the ui settings from the start menu in emulationstation

change to your new theme (you may need to press f4 or quit to refresh emulationstation in order for the theme to load)

There is a fundamental bug with EmulationStation where unique background images are not loaded dynamically so if you have more than ~10 systems (on most themes that have a unique wallpaper per system) you will start to get a white screen of death and nothing will show up.

There are a few things you can do to fix the white screen of death:

• increase your GPU/CPU split (see here)

• remove some systems

• switch your theme to any Carbon, Pixel, Eudora, Turtle-Pi, or Canela variant (as they have one static background and therefore don't crash when you have many systems)

If you can't access the theme changer in emulationstation because it's a white screen, you can ssh into the pi and change the theme name back to carbon in the backend in /home/pi/.emulationstation/es_settings.cfg

#THEME GALLERY

Access theme gallery in the esthemes menu

Theme	Preview
carbon
carbon-centered
carbon-nometa
Simple
simple-dark
color-pi
simplified-static-canela
zoid
space
simplebigart
clean-look
turtle-pi
nbba
Pixel
Material
MetaPixel
Spare

For an in depth tutorial see this thread HERE

For the official documentation on themes see HERE