Change theme for all pages using master page

There are many things you can try to solve this. Simplest one would be using HtmlGenericControl available in asp.net

Do following steps:-

1) get the dropdown values
2) Get the element of the master page
3) change the attribute of the element based on the dropdown value

Examples:-

HtmlGenericControl bodytag = (HtmlGenericControl)Page.Master.FindControl("body");

if (dropdown.value ="something")

bodytag.Style.Add("background-color","#000999?);
if (dropdown.value ="something")

bodytag.Style.Add("background-color","#000999?);
.
.
.
So on

I'm changing the color of the body tag, you can use it for any tag header, footer etc..

Also, mark your elements as server side controls like below