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This article explains how to change the theme color of the GUI, and how to list the available colors.    

From GUI:

Go to System -> Settings -> View Settings -> Theme and select the drop-down menu to see the theme colors list.

From CLI:

As this is a global change , the command needs to be run in the global mode:
# config system global
     set gui-theme ?                          <----- Here, question mark provide you the list of available themes for GUI.
          green                                        <----- Green theme.
          red                                            <----- Red theme.
          blue                                          <----- Light blue theme.
          melongene                                <----- Melon gene theme (eggplant color).
          mariner                                    <----- Mariner theme (dark blue color).
     set gui-theme red                     <----- To save the theme as Red color.
Theme for the GUI is changed to the Red Color theme.