If you’re using a desktop environment (DE) such as Xfce4 via the Desktop mode in X410, you can change its UI theme (i.e., colors, layout and etc.) by using the settings menu already built into the DE. For example, you can change the theme in Xfce4 via [ Applications ] » [ Settings ] » [ Appearance ].
If you’re using your Linux GUI apps via X410 in Windowed Apps mode and want to apply a theme, you need to manually update the system wide theme settings file or set an environment variable before launching your apps.
Setting an environment variable
GTK+ 3 (Installed themes are in /usr/share/themes)
Theme settings file for GTK+ 3
If you’re using Ubuntu, you can also set the theme for GTK+ 3 by using the ‘lxappearance‘ app. lxappearance is a desktop independent GTK+ 2 and GTK+ 3 style configuration tool from the LXDE project (http://wiki.lxde.org/en/LXAppearance).
sudo apt install lxappearance