schematic(theme): migrate project from prebuilt to custom theme

[Splaktar]

Feature Description

This actually came in through the Angular CLI feedback survey, but there is interest in having a ng generate schematic for migrating an app using a prebuilt theme to the custom theme configuration that ng add already supports.

Use Case

Many times at ng add time, users don't know why they would need a custom theme or don't know that they will need one. There are a number of gotchas and ways to get implementing a custom theme wrong. Having a schematic for this would help streamline that experience and fits well without our goals for Angular Material schematics.

It's likely that a good deal of our existing ng add logic for adding a custom theme could be re-used. It looks like the ng add logic already handles things like adding a custom theme to a project that only uses CSS (it adds a custom-theme.scss file and then adds that to the styles array in the angular.json).

There may be a need to allow the caller to point to the files where they import their prebuilt theme, as they may have moved it out of src/styles.scss. But maybe it's possible to walk the imports to find it?

Proposal

ng g @angular/material:customTheme

[devversion]

That is definitely a great idea. It would be great if we could spend some time on these things.