Color theme change in Vuetify not working

in my case i had to wrap all my components in v-app

<div id="id">
// youre code here

if you dont do this youre app use default theme.

reference from vuetify docs:

"In Vuetify, the v-app component and the app prop on components like v-navigation-drawer, v-app-bar, v-footer and more, help bootstrap your application with the proper sizing around component. This allows you to create truly unique interfaces without the hassle of managing your layout sizing. The v-app component is REQUIRED for all applications. This is the mount point for many of Vuetify's components and functionality and ensures that it propagates the default application variant (dark/light) to children components and also ensures proper cross-browser support for certain click events in browsers like Safari. v-app should only be used within your application ONCE."

the Color will not Switch

The colour of what? You don't have any components that use theme colours there. If you want to change the background colour of the toolbar for example you have to do <v-toolbar color="primary">