for heritage coaches (not Client side Human Services) you can create a coach view and drop it at the top of the coach, in the coach view you can include a copy of the modified coach_ng_controls.css, e.g. you can find out all the colors and replace them with a new color, or use a css variable for them and for different color themes change the variable at top to create a new theme with simple changes, we are including a sample modified css which will change the default blue color theme to grey theme, if certain thing don't render in IE or some other browser you can ad a " !important;" at the end of the style definition so that the override takes precedence over everything else.
Grey Theme css override