Many of the pre-made Joomla templates available today include various formatting options to improve the look of elements within your article. For example, if you wish to highlight a paragraph, you can include a style that displays the text inside a colored box.
If you would like to do this, but you don't have any pre-formatted style options in your template, then download a free extension called XTypo. Install it as normal (extensions are demonstrated in our free Joomla for Beginners course) and remember to enable it in Plugin Manager.
The available styles and instructions are displayed on the extension's page in Plugin Manager ("Extensions - Plugin Manager - XTypo"). If you wanted to use the same style as in the image above, you would enter the following into your article:
{xtypo_warning}This is an example of stylized text{/xtypo_warning}