I checked first to see if x-cart is available as a separate wordfile here: User-submitted wordfiles for UltraEdit/UEStudio

It was not. So you can get syntax highlighting on your file either of two ways.

1) You can add the extension TPL to the HTML syntax file. This is the easier way to make your change but you would need to make sure that the HTML language file does not get overwritten or you will end up having to repeat the change.

2) You can copy the HTML syntax file, rename it, change the internal name, remove the HTML specific extensions and the HTML_LANG marker. You can also add any new language specific syntax. Before making changes, read the Read-Me post in the Syntax Highlighting forum.

To copy the HTML syntax file, go to %appdata%\IDMComp\UltraEdit\wordfiles and copy the HTML.uew file as X-CART.uew (case is not important in windows file names).

To edit a syntax file in UltraEdit, got to menu -> Advanced->Configuration->Editor Display->Syntax Highlighting and choose the syntax file from the Installed wordfiles and click on the Open button. File Extensions is an entry in the first section of the file. There is a TMPL entry in the HTML language file, so you can add TPL after it.

See also File extension TPL to use HTML syntax highlighting.