Hi janjak,
You can add the following CSS code to the Backend > site template styles > ja_blockk > Tools > Custom CSS:
div[class*="header-theme-"].header-theme-dark, header[class*="header-theme-"].header-theme-dark {
background: #fff !important;
}
div[class*="header-theme-"] .navbar-brand a, header[class*="header-theme-"] .navbar-brand a {
color: #000;
}
div[class*="header-theme-"] .t4-megamenu > .navbar-nav > li > span {
color: #000;
}
div[class*="header-theme-"] .t4-megamenu > .navbar-nav > li.show > span,
div[class*="header-theme-"] .t4-megamenu > .navbar-nav > li.active > a {
color: #000;
}