The easiest method is to create an App_Themes folder within your web project. In this folder, you can add a theme and include all CSS files along with their respective image directories.
To automatically apply these styles to every page, add the necessary code to your web.config file.
Please note: This approach will apply all CSS styles universally. If you have browser-specific styles, this may not be the best solution.
[SEE UPDATED]
If you need to load a CSS file like "iespecific.css" specifically for IE 6 and not other browsers, use the following code snippet in the HEAD section of your webpage:
<!--[if IE 6]>
<link rel="stylesheet" type="text/css" href="iespecific.css" />
<![endif]-->
Similarly, for any version of IE 5 (including 5.0, 5.01, 5.5, etc), use the following:
<!--[if IE 5]>
<link rel="stylesheet" type="text/css" href="iespecific.css" />
<![endif]-->
To target specific versions such as IE 5.5, you can use the following code:
<!--[if IE 5.5000]>
<link rel="stylesheet" type="text/css" href="iespecific.css" />
<![endif]-->
For example, to apply styles only to IE versions greater than or equal to 6, use:
<!--[if gte IE 6]>
<link rel="stylesheet" type="text/css" href="iespecific.css" />
<![endif]-->
These code examples work because regular browsers interpret the block as HTML comments, while IE parses them for specific instructions.
You can also exclude certain style sheets using a similar method. To exclude "not-ie.css" from IE 6, use:
<![if !(IE 6)]>
<link rel="stylesheet" type="text/css" href="not-ie.css" />
<![endif]>
By using the NOT operator, the styles are applied only if the browser does not match the specified expression.
Remember that while this method works effectively, it may not pass HTML validation as it utilizes non-standard tags.
If you want to load a style sheet only if the browser is not IE 5 or above, you can use:
<![if !IE]>
<link rel="stylesheet" type="text/css" href="not-ie.css" />
<![endif]>
For more information on this feature supported by Microsoft, refer to this documentation link.
While the documentation doesn't specify compatibility with Macintosh versions of IE, it's reasonable to assume that these features extend to those platforms as well.