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The big issue with XOOPS theming is that it's very complex to do so.
What do I mean with that? XOOPS is burdened with the past, a lot of modules that are used often are quite old and are built years ago, and present all of their data and layout into table structures. However, since more than 10 years ago, designers have moved away from tables and more into div's and fluid layouts.
Tables are great for displaying data, but for layouts they're really the worst thing you can imagine. And that's a huge issue if you want to design for XOOPS: you have to keep in mind that almost everything is tabular, and if you want to work around that, you're basicly forced to write custom templates for every module, and there are a lot of those around.
The big issue with this is that in many themes, many modules render badly or are very hard to customize. Another thing which hurts us is that there isn't a "straight coding guide/style guide", every module developer has his own style and way of working. What this means is that every module looks different and is hard to style.
On WordPress, you don't have this issue: it's a blog by nature but can be used as a CMS, and in general plugins style themselves to look "okay" or "good". We don't have that here. A module can be shown awesomely in 1 theme, but look horrible in another one. We basicly need 1 or 2 theme frameworks to which all modules are written, but it will be a huge task to rewrite everything to that new standard/framework. A lot of work has been done in xBootstrap, but there's still a lot to do there. I've been working for weeks on a port of Foundation to XOOPS and it's really staggering how much work you need to do to get things to look right. You can port it over to XOOPS easily, but making it look Okay is very hard.
Currently, for me, the biggest issue in theming is not that it is too hard, but that it is way too much work to support all of the modules. We need to port those over to Bootstrap (not in the themes, but in the modules themselves!), and should push for new modules to be based on bootstrap themselves too. That way it's going to be a lot easier to theme stuff: if module developers design their modules using default Bootstrap elements, we just need to modify those default elements. Right now every module does things their own way, which is bloody hard to override.
As for documentation and the status of that, I'm working with Mamba (Michael) to get things done there too, but the wiki on XOOPS has quite a few issues and doesn't allow image uploads so that's blocking our progress. We're looking into a solution for that now.
The idea that I have is to have a wiki that everybody can edit, but only focussing on English and to have a basic article ready about everything and expand later. But first, a good foundation to work on and we're ready to go a long way.
I'm also a fan of a redesign of the XOOPS website (the current design dates from 2009! already, it can use a few updates). But once again, the modules underneath need the real update... :P If you want to know what hard is, try to theme the forums. It's a big pain to do so!