Update assistant theme

[marcoambrosini]

Describe the feature you'd like to request

• Change the selected tab to primary button style
  Component is here: feat(NcAssistant*): provide components for consistent integrations design nextcloud-libraries/nextcloud-vue#7398

• Use --color-element-assistant-icon for the Nextcloud assistant title. It's ok to assign the same background to the icon and text just like in the assistant secondary button.
  Varialbes: Add variables for Assistant colours server#54665

Lib version shipping the change is 8.30.0
https://github.com/nextcloud-libraries/nextcloud-vue/releases/tag/v8.30.0
https://www.npmjs.com/package/@nextcloud/vue/v/8.30.0