title: css is lost on postback
body: <p>I currently have a problem with my website in ASP.net / C#. I'm building a page, where a couple of elements are dependent on others. On this page I use some import css classes, though they seem to get lost, whenever a Postback happens. Could anyone take a look at the code and tell me where it goes wrong? </p>
  
  <p>I used a .NET fiddle file to paste the code of my .ascx and .ascx.cs file. I hope it's somewhat clear. Apologies if it's not. </p>
  
  <p><a href="https://dotnetfiddle.net/NLwtiN" rel="nofollow noreferrer">https://dotnetfiddle.net/NLwtiN</a></p>
  
  <p>EDIT: I tried to compare the html of 2 DropDownLists, hoping there'd be some significant difference, though there is next to none </p>
  
  <p><img src="https://i.imgur.com/Dy2bKHD.png" alt="image"></p>