2011 version of WP?

You’ll need to be an Administrator on the site to modify or change the theme: https://www.ads-software.com/documentation/article/roles-and-capabilities/#capability-vs-role-table

So, if you have access to an Administrator account, you’ll need to log in under that and you’ll find everything you need under the Appearance section of the Dashboard: https://www.ads-software.com/documentation/article/appearance-themes-screen/

If you don’t have access to an Administrator account, you’ll need to ask one of the site’s Administrartors to promote your account: https://www.ads-software.com/documentation/article/users-screen/#change-roles-to

twenty eleven is a classical theme but it wouldn’t have any effect on wordpress core. What version of wordpress are you running ? Also The apperence option is on the Left hand side of the panel. It changes based on if you are running a classical theme or a block based theme. Be careful of just switching themes between a classical and block theme. Take time to stage the new theme before changing. There are a number of things that could go wrong with just changing themes out. For instance if you are running widges in the classical theme they will not work in the block based theme.. Also, you should be able to change background colors using the custom css option in the classical theme. Be careful not to cause more problems then you want to deal with.