Modify or Create a CSS class in C#

Probably the best thing to do is create an aspx page with or without a codebehind file, then just do your css manipulation there. Here's a basic example.

<%@ Page Language="C#" %>
<%
    string color = "blue";
%>
H1 { background-color:<%= color %>; }


Then reference that file in your other pages just like you would any other css file.

<link type='text/css' rel='stylesheet' href='../myCSS.aspx' />

As an example:
<element name>.Style["width"] = "1000px";

@Teggert

I Like your suggestion, but I cant get my page to recognize the css in an aspx page

<link type='text/css' rel='stylesheet' href='../myCSS.aspx' />

I added this to my page, but no styling appears.

My myCSS.aspx looks like this:

<%@ Page Language="C#"%>


<%@ Page Language="C#" %>
<%
   string color = "#123456";
%>


      /* Remove margins from the 'html' and 'body' tags, and ensure the page takes up full screen height */
html, body {height:100%; margin:0; padding:0;}
/* Set the position and dimensions of the background image. */
#page-background {position:fixed; top:0; left:0; width:100%; height:100%;}
/* Specify the position and layering for the content that needs to appear in front of the background image. Must have a higher z-index value than the background image. Also add some padding to compensate for removing the margin from the 'html' and 'body' tags. */
#content {position:relative; z-index:1;padding:10px;}
       
        .grid
        {
            border: solid 2px <%= color %>;
            font-family: Tahoma;
            font-size: 12px;
            width: 700px;
            background-color: #fff;
            margin: 0px auto;
            position: relative;
            margin-top: 50px;
            padding-bottom: 5px;
        }
        .grid H2
        {
            background: #8988a5 url(../images/sgtitle-bg.gif);
            border-bottom: solid 1px #000;
            height: 28px;
            line-height: 28px;
            margin: 0px;
            padding: 0px 0px 0px 5px;
            color: #000;
            font-size: 15px;
        }

Just make sure the location of your myCSS.aspx file is correctly referenced. If it's in your site root, just do '/myCSS.aspx' or 'myCSS.aspx' instead of '../myCSS.aspx'

Its at the root. Im adding it dynamically, like i was doing for the other stylesheets:

           var link = new HtmlLink();
            link.Attributes.Add("href", Page.ResolveClientUrl("myCSS.aspx"));
            link.Attributes.Add("type", "text/css");
            link.Attributes.Add("rel", "stylesheet");
            head.Controls.Add(link);

Give this a try.

            var link = new HtmlLink();
            link.Attributes.Add("href", Page.ResolveClientUrl("~/myCSS.aspx"));
            link.Attributes.Add("type", "text/css");
            link.Attributes.Add("rel", "stylesheet");
            Header.Controls.Add(link);

I tried that first, no luck. I even put the link directly on the page. No styling. My old cole looked like this:

link.Attributes.Add("href", Page.ResolveClientUrl("~/Styles/" + site + "Style.css"));

where the variable site was the client the user selected. This worked fine. I moved myCSS.aspx under the styles folder and updated my code

link.Attributes.Add("href", Page.ResolveClientUrl("~/Styles/CSS.aspx"));

No styling :(

Yes its CSS.aspx not nyCSS.aspx. I just called it myCSS.aspx to be consistent with what you called it

That fixed it!!!! Man you're a genius. I had no idea you could use an aspx page as a stylesheet. Thank you so much! 500 points for you

Wonderful

No problem man. Glad it worked out. Feel free to post any other problems you run into here and I'll help out as I have time.