Use core theme as default Reader theme if installed (as opposed to legacy)

[westonruter]

Feature description

When the AMP plugin is activated and a core theme (or another AMP-compatible theme) is active, the AMP plugin defaults to Transitional mode since the theme is AMP-compatible:

amp-wp/includes/options/class-amp-options-manager.php

Lines 105 to 122 in 85f53e6

	// Migrate legacy method of specifying the mode.
	if ( ! isset( $options[ Option::THEME_SUPPORT ] ) && $theme_support ) {
	$template = get_template();
	$stylesheet = get_stylesheet();
	if (
	// If theme support was probably explicitly added by the theme (since not core).
	! in_array( $template, AMP_Core_Theme_Sanitizer::get_supported_themes(), true )
	||
	// If it is a core theme no child theme is being used (which likely won't be AMP-compatible by default).
	$template === $stylesheet
	) {
	if ( empty( $theme_support[ AMP_Theme_Support::PAIRED_FLAG ] ) ) {
	$defaults[ Option::THEME_SUPPORT ] = AMP_Theme_Support::STANDARD_MODE_SLUG;
	} else {
	$defaults[ Option::THEME_SUPPORT ] = AMP_Theme_Support::TRANSITIONAL_MODE_SLUG;
	}
	}
	}

When the active theme is not AMP-compatible, then the plugin defaults to Reader mode, and specifically the "AMP Legacy" theme. The result is a default experience that uses our least-preferred Reader theme. We should rather check to see if a core theme is installed (as they often are) and if so, use the latest one as the default Reader theme.

So if a site has a custom theme active (which is not known to be AMP-compatible), but they also have Twenty Seventeen, Twenty Nineteen, and Twenty Twenty installed: then the default Reader theme should be Twenty Twenty since it is the most modern.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Related to #1384 which is about defaulting to Standard mode when the theme and plugins are all AMP-compatible. That issue was also originally about defaulting to Transitional mode when the theme is AMP-compatible, but that is now implemented.

[jwold]

Based on our discussion:

Weston:

If a user has multiple Core themes installed it's difficult to identify the best one Core theme for them. The simplest method is to choose the most recently published. Twenty Twenty out of the box has more options out of the box. If we do always go with the most recent theme we may want a default configuration we automatically supply that normalizes the style.

We can go with reverse chronological (and offer override config), or we can go by preference, likely choosing Twenty Twenty first.

One more thing to consider, sometimes Reader Themes don't work because they have a Custom Post Type defined in the theme itself, or a recent case where a plugin was active, depended on a theme being active, and the plugin tried to call a function defined in the theme. That caused a fatal error, in that case using legacy is better.

[westonruter]

If someone has a custom post type or something else defined in the active theme itself, then the legacy theme should continue to be used.

We need to do a theme compatibility check per #4795 for this as well.

[jwold]

New theme selection in reader mode onboarding

We would like to limit the Reader theme selection screen (in the onboarding and AMP settings page) to 3 Core themes + the legacy theme.

Ordered: Twenty Twenty One, Twenty Twenty, Twenty Nineteen, AMP Legacy.

Then after those 4 we'll show installed themes.

Add documentation knowledge path

Within the theme selection screen of onboarding (and settings), add link to documentation / themes ecosystem page on amp-wp.org.

Default theme

Make Twenty Twenty One default theme.

---

Suggested order:

Rough sketch of the notes above:

cc @amedina and @westonruter

Relates to #6036

[westonruter]

Slight tweak to the order. I think it should be: 2021, 2020, 2019, followed by any installed core themes (or other AMP-compatible ecosystem themes) and then lastly put AMP Legacy, since it is the one we want to discourage use of the most.

The one caveat here is that Reader themes may not be available at all for the conditions outlined in #4795 (comment), but this would have already been accounted for.

[jwold]

Action items from our 2021-04-13 Call:

• Limit default to show 3 core themes on Vanilla WordPress. Then show the ones that are actually installed. Order by date: 2021, 2020, 2019.
• Link to documentation on amp-wp.org for adding Reader themes; entry point for knowledge path.
• Link to themes ecosystem page as well. May hardcode those into the plugin (not flagged auto yet).
• Default to 2021 as Reader theme as opposed to amp legacy.

Relates to #6036

[westonruter]

This is also somewhat related to #2313. With highlighting these core themes as Reader themes, we could eventually list other key themes from the ecosystem page that work great as Reader themes,

[westonruter]

After thinking about this some more, I don't think we should change the default theme we activate because we need the theme scanning results in order to safely do so (#4795). Also, we want to eventually make Standard mode the default for new activations. Therefore, any change here would be just changing the order of presentation for the Reader themes. So I'm going to close this for now.