theme injecting css into wp-admin

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The problem is that you are enqueue the CSS file of your theme outside any of the recommended actions. If you want it to be added only in frontend, you should use wp_enqueue_scripts action:

add_action( 'wp_enqueue_scripts', function() {
    wp_register_style( 'mainstyle', get_stylesheet_uri());
    wp_enqueue_style( 'mainstyle', get_stylesheet_uri());
});