I just use the ASPX method of getting a full path with dynamic context and path control with ~/App_Theme/...

That only works if it is in code that gets processed either directly by controls that support it, like ASP:Hyperlink NavigateUrl="~/..." or in any code that is processed by the Kentico Resolve filter. (See the Developer's Guide -> Page processing and URLs -> Output Filters page for more info.)

For example, that won't work in external JS files, but it will in portal pages.