check the theme customiser for ‘theme color’;
you could also try to add some new CSS via the ‘cusom CSS’ section of your theme; example:
.secondary-navigation,
.secondary-navigation ul ul li,
.secondary-navigation ul ul li:hover,
.secondary-navigation ul ul ul li:hover,
.secondary-navigation ul ul ul li,
#mobile-menu-wrapper,
a.sideviewtoggle
{
background-color: green;
}
Thank you so much! It was in fact in Appearance > Customize > Design & Layout.