Theme development

OK, I'd think this should be an easy one, but I've searched, and been unable to find anything on this, at least as of yet.

I'm creating a theme, and I would like for it to be compatible with both Drupal 8 and Drupal 9. I'm unable to find how I would specify the

core:

line in the info.yml file to indicate 8/9 compatibility. I know it would be

core: 8.x

for Drupal 8, and thus, presumably 9.x for D9, but I'm not sure of the syntax to indicate both.

Hello,

I have installed Drupal 9 locally. I've installed Bootstrap Barrio theme and created a sub-theme. I tried to enable Twig Debugging in services.yml file as shown on these instructions https://www.drupal.org/docs/theming-drupal/twig-in-drupal/debugging-twig..., but I do not see the comments in the source. Enabling twig debugging in my drupal 8.x sites works. What am I doing wrong? 

Thank you.

Hi,

Not sure if this is the right place to pose the question but here it is anyway.

I am learning the Umami distribution and was playing around with the front page views and the theme regions. I wanted to know in which region the default view page appears. This was not clear so I started to delete regions from the umami theme info file. I got to the point where I deleted all of the regions but still the view page output appeared on the front page.

Hello,

I am working on my second website and have somehow broken it and had intermittent access yesterday, and what seems to be no access today. I suspect it is a DNS error, as I sometimes can't even access Cpanel, but I wan't to eliminate all variables.

Hi! How can I set name to page template depending on the type of material in drupal 9? Thanks.