To dynamically load the style sheet in the content, you must first set the link as a server control and dynamically bind the href of the link tag in the background to dynamically load the style sheet of the content page.
Master page:
<Link type = "text/CSS" rel = "stylesheet" href = "CSS/basic.css" id = "CSS" runat = "server"/>
// The href on the master page can be written or not written, but you need to add runat = "server" to the link label, and set the link ID so that it can be called in the background.
Content Page:
Htmllink CSS = Master. findcontrol ("CSS") as htmllink;
CSS. href = stylesheet;
// Stylesheet is the address of the style sheet on the content page, which is relative to the address of the root directory. For example, if stylesheet.css is in the CSS folder under the root directory, you must write "CSS/stylesheet.css ".