Can you use an external style sheet or an internal style sheet in this situation? What to do with it

Can you use an external style sheet or an internal style sheet in this situation?
I have a menu.php, menu.php defines the navigation menu part of the left and right pages, that is, the head tag pair and body tag are in menu.php, the head tag pair uses an external style sheet style1.css, and the other PHP require menu.php.
For example, customer.php require the stype of those elements in menu.php,custom.php in a non-attribute menu.php can only use inline style sheets? Or can you define the STYLE1.CSS?
I defined the class attribute for the HTML tag in customer.php, and I set the CSS property on the class in Style1.css, and the result doesn't work. Should it work?


------Solution--------------------
There should be other reasons for it!
------Solution--------------------
At least you're not clear about the problem you're having, style has an inheritance, and that's what everyone knows.
------Solution--------------------
Supposed to work.
I think you should check if the stylesheet is loaded and the href is correct. If you use Chrome, the review element feature can help you see all the style attributes

