It entirely depends on what you mean by designing a user interface...
If you are talking total overhaul of the interface and all functions then yes PHP is a must. PHP is the heart of Moodle.
However, if you're talking about just a front end or changing the way it looks, so that Moodle looks better than that, frankly annoying as hell, list of categories and subjects then HTML and CSS will do you.
It's the difference between constructing a whole new interface and applying a new theme.
The CSS is primarily affected by the theme you apply to your site, so the CSS files you need to change/edit are in that theme not in central Moodle.
I personally exchanged our front end for a more graphical version. I got the school graphic designer to create me some logos and then used them as buttons with hover effects, have a look at https://moodle.towers.kent.sch.uk if you want to see what I mean. If you hide everything else, I even defaulted the admin blocks to be docked, then you get a nice graphical front end. It has actually improved the way the students access Moodle immensely. It's only for the time being though because the school, in their wisdom(?) are scrapping Moodle. But that said I shifted the icons into sprites so less graphic loading was involved (otherwise it was 104 images to load each time). Our Moodle also runs as an internal/external split site so I redesigned the internal interface with a version 2, something a bit more complex. The internal version also used HTML5 animation effects, CSS, AJAX and JQuery mobile to develop a more tablet friendly interface.
Either version used the same approach though you add a topic section to the front page through the front page settings and then add an HTML label into that topics section then it's just like a normal web page.