Try to add the following code snippet to the Custom CSS area in Theme Settings to change dashboard colors.

.dokan-dashboard .dokan-dash-sidebar ul.dokan-dashboard-menu {
	background-color: black;
}

.dokan-dashboard .dokan-dash-sidebar ul.dokan-dashboard-menu li.active,
.dokan-dashboard .dokan-dash-sidebar ul.dokan-dashboard-menu li:hover,
.dokan-dashboard .dokan-dash-sidebar ul.dokan-dashboard-menu li.dokan-common-links a:hover {
	background-color: #2678CC;
}

.dokan-dashboard-wrap .dokan-dashboard-content .dokan-btn-default, 
.dokan-dashboard-wrap .dokan-dashboard-content .dokan-btn-sm, 
.dokan-dashboard-wrap .dokan-dashboard-content .dokan-btn-theme, 
.dokan-dashboard-wrap .dokan-dashboard-content button:not(.dokan-close), 
.dokan-dashboard-wrap .dokan-dashboard-content input[type=submit], 
.dokan-dashboard-wrap .dokan-dashboard-content input[type=button] {
	background-color: #2678CC;
	border-color: #2678CC;
}

Regards