Enqueued Stylesheets Effecting Admin Styles

You could do it like this:

function theme_styles(){
    /*
     * This if() statement is unnecessary, as wp_enqueue_scripts
     * doesn't fire on the admin pages.
     * if( is_admin() ) {
     *   return;
     * }
     */
    wp_enqueue_style(
        'theme-styles', 
        get_template_directory_uri() . '/css/all.css', 
        array(), 
        false,
        'all'
    );
}
add_action( 'wp_enqueue_scripts', 'theme_styles' );

References

• is_admin()

Also, note that the wp_enqueue_scripts hook is used to enqueue both scripts and styles.