As mentioned before, packaged themes can include custom block view templates, but they can't override the default view templates of Concrete CMS core block types. Unfortunately, this means it would be very difficult for me to package the theme demonstrated in Modifying the Default View Template for a Core Block Type. Why? Because my custom view template can't override the Image Slider block view template when used in a packaged theme.
Or can it?
While it's true that Concrete won't look for an overridden view.php file for a core block type in a packages blocks directory, you can get something almost as good. You can create your overriding view template as a custom template, and then instruct the theme to use that custom template when adding new versions of that block type. That way, you don't have to instruct the users of a theme to use a custom template rather than the default view.
How does it work? Let's use the Image Slider customized default view template that we worked with earlier. That view template is currently at application/blocks/image_slider/view.php. Let's say that I want this view template to be used by the Image Slider block any time it is used inside my theme, which is named "Enterprise".
First, create a custom template, packaged in my theme's package directory.
mkdir packages/enterprise/blocks
mkdir packages/enterprise/blocks/image_slider/
mkdir packages/enterprise/blocks/image_slider/templates
mv application/blocks/image_slider/view.php packages/enterprise/blocks/image_slider/template/enterprise_image_slider.php
At this point, I've created a custom template named "Enterprise Image Slider" that, when used, will display nicely in the Enterprise theme. Unfortunately, you have to explain to your users that they need to do this. Worse, until they do so, they'll get weird looking image sliders on their pages. Thankfully, it's not hard to default the Image Slider block to use this custom template when it is added to the Enterprise theme.
My packaged theme needs to have a PageTheme class. If you're unfamiliar with this concept (which is completely new in 5.7) you should check out Enabling Grid Support for Areas and Layouts, which describes it fully. Basically, a PageTheme class is an additional code file that documents certain capabilities and attributes that a theme has. This custom template default is just another one of those attributes. In our case, once we've created a PageTheme class for our Enterprise theme, we just add the following code to it:
public function getThemeDefaultBlockTemplates()
{
return array(
'image_slider' => 'enterprise_image_slider'
);
}
And that's it! You still have your custom template, but any time the Image Slider block is added to a page using the Enterprise theme, the "Enterprise Image Slider" custom template will be used instead for its view template.