Once you’ve chosen your theme you are probably going to want to change it here or there. May be the font isn’t to your liking, or the colours needs to be changed slightly. As you go on you might want to re-style a particular element, or add some more space between elements.
One of the first things that you notice in the Appearance section of the WordPress Dashboard is the Editor, the temptation is strong to click on it. If you do so in the default theme you get a screen that looks something like this:
Down the right hand side you will see a list of Templates, and at the bottom, Styles. These are majority of the files that make up the theme for your site. In the main part of the screen the text box allows you to change whatever template or style is selected.
However, you should never change the theme in this way. The reason is simple, if the theme is updated at some point in the future, you will either need to migrate your changes into the new version of the theme, or not update to the new version at all. There is a better way: Use a Child Theme as described in this WordPress Codex article.
The article goes through the process but, basically, you create a directory on the server and write a small file called style.css into that directory. When you go into the Themes section you should now see a new theme with the name that you gave it as detailed in the article. Now you can change the theme to change the fonts or colours without affecting the original theme’s files.
There are some complications. Firstly you have to get access to the server that’s hosting your blog, normally you should have ftp or sftp access, if you don’t you may need to check your providers FAQ’s to see what the alternatives are. The nasty problem is that some themes use clever methods to build their style sheets which mean that they cannot be used as the parents for Child Themes.
For more information you should look at the following resources: