The thing with SMF theming (and this has been the main thing discouraging me from doing anything in-depth) is that they do not have any sort of disconnect between form and function. So as we continue using this theme (which IIRC was based on SMF 2 RC2 I think?) and the forums continue to be updated with new patches, there will be a growing disconnect between the functionality provided and expected by the forum code, and the features that the theme has built in.
I mean no disrespect or offence, and this post is not just about SMF themes but in general. If you don't have time, will or both to keep up with web application updates, then perhaps you should admit as much and request to have someone on-board who does. See, themes can have bugs and some of them may even be security related. So keeping themes up-to-date is important too. As a neat side-effect, custom theme stays consistent with default theme and there shouldn't be weird layout or color issues like there currently is in the HLP theme.
Another thing to consider is, perhaps you're customizing HLP theme more than is necessary. Sure it is nice to have something different, some bells and whistles and some additional eye-candy, but in the end they cause nothing but additional work when you have to update theme to latest version.
Another thing to remember is that most web applications supply detailed changelogs. Which is excellent means to check if you need to update files, themes or otherwise. In that case you can simply run side-by-side code comparison to see what needs to be modified, added or removed. Reducing time needed to keep things up-to-date.
Bottom line is, if it is true that the current theme is still based on SMF 2.0 RC2, then that means there might be potential security holes in the theme files themselves. It's no longer just an inconvenience from some layout/color issues, but security as well. Same applies to MediaWiki and anything else.