How to fix a theme for the website in WordPress?

I have a WordPress website hosted with Bluehost that consists of few pages, in particular a home page and few article pages (posts).

From the WordPress dashboard I go to Appearance and then under the themes I activate a theme called "twenty nineteen". If I load my website all the pages seem to use this theme.

The problem is that if I then go back to the dashboard and edit one of the pages, the theme that I applied earlier is not used anymore and I need to repeat the activation from the Appearance section in order to reapply the theme I want.

Is there a way to fix a theme so that even if I edit the content of my posts I won't lose the theme that was chosen in the Appearance section?