Node view mode in page template suggestions I would like page template suggestions when a node is using a custom view mode.

I think this would happen in hook_theme_suggestions_page_alter().

I have already been able to get page-level template suggestions based on the node's content type, so it seems like I should be able to snag the name of the custom view mode?

I'm not good at this part of Drupal at all, so any help would be appreciated.
The hook hook_theme_suggestions_HOOK is the one you should implement. 

When you have a node of type page (labeled as Basic Page from the default profile), then the HOOK is node and the specific hook you would implement is hook_theme_suggestions_node(array $variables)

As @pinueve points out, there is already a specific suggestion that includes the custom display mode: node--page--CUSTOM_VIEW_MODE.html.twig.

Also, as @pinueve suggests, the easy way to find the template suggestions is by enabling the debugging mode. I tested this using a custom view mode called customvm that I created, so I can see these suggestions in the browser by inspecting the source code:

* node--17--customvm.html.twig
* node--17.html.twig
* node--page--customvm.html.twig
* node--page.html.twig
* node--customvm.html.twig
x node.html.twig


Alter hook

Note that there is also an alter version of the hook [HOOK_theme_suggestions_node(array &amp;$suggestions, array $variables)][3], you should use this if you are trying to alter the existing suggestions, maybe changing the priorities.

Suggestions for a page from a custom controller

The page HOOK is meant to be used when the output comes from a custom controller and route, like the one in this example:
https://www.drupal.org/docs/8/api/routing-system/introductory-drupal-8-routes-and-controllers-example

In this case the HOOK is page and you would implement these hooks: 


HOOK_theme_suggestions_page(array $variables)
HOOK_theme_suggestions_page_alter(array &amp;$suggestions, array
$variables)