The problem is that the $opt_val global variable is being set only when the my_plugin_options function is called. And that function is being called only in case user is viewing the My Plugin Options screen.
You’ll have to call get_option( 'mt_favorite_color' ); inside the hook_css function. (Please note that the hook_css function is not being prefixed. All custom functions, global variables, options etc really shold be prefixed).
Further, the code you shared is not sanitisating values on user input, nor escaping theme on the output. Make sure to read Validating Sanitizing and Escaping User Data part of the codex. and use sanitize_text_field before saving the value to the database, and esc_attr when outputting the value in order to make the plugin more secure.