How to reset display of WYSIWYG editor

Looks like the Theme you are using adds style to the editor using add_editor_style().

Check for that code in theme files and just comment it out by adding a // so that it looks like //add_editor_style( ... );.

Alternatively, you can look for a file named editor-style.css in theme’s root directory and rename it to something else.

EDIT:
Yes, it does add editor style. You can comment out the line# 326 in functions.php to use the default WordPress styles for Editor.

//add_action( 'init', 'gravit_add_editor_styles' );