[Feature Request] Allow administrators to configure the global color palette used by Nextcloud apps

Is your feature request related to a problem? Please describe. Currently, many Nextcloud applications such as Calendar, Deck, Mail, Tasks, and others use a shared predefined color palette provided by the common color picker component. While administrators can customize branding-related colors through the existing Theming settings, there is currently no way to modify the predefined color palette itself. This can be problematic for organizations that need to align all colors with corporate branding, accessibility requirements, or other deployment-specific design guidelines. As a result, users are presented with colors that may not match organizational standards, and administrators have no central way to influence the palette used across applications. Describe the solution you'd like Extend the existing Administration → Theming settings with a new section that allows administrators to configure the global color palette used by Nextcloud applications. Possible functionality: &…