Theming contact page form
Way 1: Use drupal template suggestion:

You can enable the template suggestions by following changes as mention in 
https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates

Once you have the suggestions enabled you can see, drupal suggests few template possiblities for any page, you can use any of the suggested template.

https://cl.ly/1f2T0x1a1F2n

Way2: Declare custom template suggestions

Or you can customd define template based on any logic by implmenting hook_theme_suggestions_page_alter().

Example:

https://sqndr.github.io/d8-theming-guide/theme-hooks-and-theme-hook-suggestions/theme-hook-suggestions.html