Working fine for me. Likely a cookie issue so reset your Cookies for EEVBlog make the changes you want and it should be fine.
However. As the theme it is just a stock off the shelf one it is missing some of the extras applied to the base theme that the Forum has evolved on.
It is a Major PITA to get two different themes to follow changes to the base forum. Even creating a Dark look of the main theme is not trivial. Adding a theme change button is possible down the bottom but as it is not in the Core of SMF every time you get an update to the software fairly likely you would also need to tweak the mod depending on how it was coded.
Don't ask me how I know but I added to the hair loss and Gray colour of the remaining hairs elsewhere