Showing "New content" form on main theme
In Drupal 8 there is the entityform_block module which gives you the option to display an entity form as a block. 
You just need to go to Blocks, select Place block in the region you want and select Entity form.
When you add your from you can select Entity Type + Bundle, where you have to select your content type and also in which pages you what this block being visible.

Keep in mind that using this way, on submit the user will be redirected to the content page but you can write your custom handler using hook_form_alter.