Every major CMS comes with a default theme. Do not use this default theme for your website. Ever. Thousands of other sites already do.
Instead, use one of the many free themes available for your CMS.
Remember WordPress Blue?
Default themes are a plague of the Internet. For instance, I bet you recognize that little blue WordPress theme, don't you?
The official caption for this theme reads, "The default WordPress theme that graced version 1.5 to version 2.9, based on the famous Kubrick."
My caption would read, "This was a nice theme. It really was. The first three thousand times."
Depending on your browsing habits, there were a few years there where it seemed like every other site was blue.
(Yes, mine was one of them. Briefly.)
Today, WordPress has a new default theme, but the old one lives on. Do an image search on "wordpress default theme", and you'll see what I mean.
By now, this theme is not only a (continuing) bad advertisement for WordPress, it's also a banner that shouts, "Greetings! I probably haven't gotten a security update since the Lord of the Rings movies! Try me!"
Please don't use the default theme.
Finding a Free Theme Is Easy
Don't think that a quality theme has to cost money, or require a web designer. WordPress, Joomla, and Drupal all have a large community of theme developers who have shared their work for free.
For WordPress and Drupal, you can browse an official directory of freely licensed themes:
In Joomla, "themes" are called "templates." Sadly, there doesn't seem to be an official free template directory on Joomla. Instead, they link to template providers.
Some of these providers offer free themes, such as the excellent Quasar template.
But you'll probably have to search for "free joomla templates," and have a careful look around. There are plenty of good, free templates for Joomla. Just make sure your template is not only free of cost, but also has a free license, like the GPL. This guarantees that you can make any changes you like.
You Can Often Customize a Free Theme
A few of the free themes out there may seem almost as popular as the default theme. If you like a theme, but you feel like you've seen it before, don't worry. You may be able to customize it.
In fact, you can even customize the default theme. But you're usually better off finding a theme that is designed to be customized.
If you're comfortable with CSS, you'll definitely be able to customize anything you find.
Just make sure your changes won't get overwritten when you upgrade the theme. A good theme is careful to provide a separate file where you make your changes, so they stay safe.
Getting a Custom Theme May Be Worth the Cost
I build websites, so the people I work with are usually willing to pay for a customized theme. They want a unique look, and they've often crafted that look with a designer. If you feel the same way, the expense (in money or time) will be worth it.
But if you don't have a definite vision for your site, browse the free themes out there, and pick one. You can easily find a good one, and almost anything you choose will be a huge improvement over the default.