Hello sorry i speak english not very well, but i think read manuel https://github.com/recalbox/recalbox-os/wiki/Manual-(EN)#recalbox-33x Special Commands: https://github.com/recalbox/recalbox-os/wiki/Manual-(EN)#b---special-commands

In game Press Hotkey on the controller and one of the following: R2 → Next shader preset L2 → Previous shader preset

You can access retroarch configuration menu with Hotkey + B If you want to configure retroarch and save the config, you can select the "Save Settings on Exit" in the retroarch menu. After that, all configuration you make in rgui will be saved.