With WordPress, the style of your website is arranged in a theme, with Joomla in a template. You can choose from thousands of options for both systems, from free to very expensive. They range from very simple to very extensive, from old-fashioned to hip and from very ugly to very beautiful. This applies to both WordPress and Joomla: with both systems you can create an equally beautiful website.
Differences WordPress and Joomla
Only one theme can be used at a time on a WordPress website. Joomla allows you to assign templates and template styles to pages, so you can use different templates or styles for different pages.
You can easily copy a template style in Joomla. This is not possible in WordPress, you have to create a child theme and then work in it.
Also different is what is arranged in your template or theme. WordPress often includes extra functionality in the themes, for which you would otherwise use a plugin. Handy, you might think… until you want something different and install a different theme. Get rid of the contact form, slideshow or whatever nice extras your previous theme had included.
You will want to adjust the styling of your website faster than the functionality, so it is smarter to use a plugin for that functionality and leave the design to your theme. It is important that your theme supports the functionality of your plugin. Otherwise, choose either a different theme or another plugin that works with your theme.
There are also templates with built-in extras for Joomla, but that is less common. Extensions for Joomla are generally built to work well with almost all templates. A good starting point in your search for a Joomla template is Themeglobe.
How do you choose a theme or template?
Not based on the extras :). The more extras, the more scripts and the heavier your theme or template. But what do you pay attention to?
If you are not so proficient in CSS, you could look at a theme or template in which you can adjust colors, borders etc. And to a theme or template in which the design has been consistently implemented.
In any case, do not be fooled by the image of the demo. Theme and template developers often use razor sharp, beautiful photos. With your own photo it suddenly looks a lot less fancy. Moreover, those razor-sharp photos are also very heavy and that affects the performance of your site.
So don't use your theme or template as a starting point. That's the outside. First make sure that all functionality is working, and then throw a nice wallpaper over it.