This article explains how to change the theme color of the GUI, and how to list the available colors. Solution
Go to System -> Settings -> View Settings -> Theme and select the drop-down menu to see the theme colors list.
As this is a global change , the command needs to be run in the global mode:
# config system globalTheme for the GUI is changed to the Red Color theme.
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.