This gives you the option to select a and set a new theme for your system.

Compatibility issues for themes are not unheard of, so be sure to make sure there are no reported issues on the theme you’re applying.

A selection of Batocera-compatible themes can be installed through the menu SYSTEM SETTINGS → UPDATES → THEMES.

This menu gives you a list of themes available like:

[A] Fundamental - https://github.com/jdorigao/es-theme-fundamental
[I] Zoid - https://github.com/RetroPie/es-theme-zoid
[I] Carbon - https://github.com/RetroPie/es-theme-carbon
[A] Art-book - https://github.com/anthonycaccese/es-theme-art-book

The first 3 characters indicate if the theme is:

• [A] Available for install (i.e. not installed on Batocera yet)
• [I] Installed (but can be re-installed, in case there is a new version for example)
• [?] Unknown status

You can override the default theme selection and add your own themes by writing your own /userdata/system/themes.txt file following this format:

Fundamental      https://github.com/jdorigao/es-theme-fundamental
Zoid             https://github.com/RetroPie/es-theme-zoid
Carbon 		 https://github.com/RetroPie/es-theme-carbon
Art-book    	 https://github.com/anthonycaccese/es-theme-art-book

It very easy from a user perspective, click on the following picture for an animated GIF describing the whole process to install a new theme and switch it on under Batocera: