When you use a default WordPress themen and you want to make some adjustments it’s not always the best solution to just go and make modifications to the style.css.
For example, when you install an important update of the theme you use, your modifications will be lost.
Create a child theme:
Create a folder in the themes directory (wp-content/themes)
Name the folder the same as the ‘parent’, if your parent theme is called theme1 call your child theme folder: theme1-child.
In the folder theme1-child create a file; style.css.
Edit the file and add this:
Theme Name: Theme1
Theme URI: http://example.com/theme1
Description: Twenty Fourteen Child Theme
Author URI: http://example.com
/* =Theme customization starts here
You can change each of these lines to suit your theme. The only required lines are the Theme Name, and the Template.
The child theme’s stylesheet is included after the parent theme’s and styles will therefore override those in the parent theme’s stylesheet.
Activate the child theme. Log in to your site’s dashboard, and go to Administration Panels > Appearance > Themes. You will see your child theme listed there. Click Activate.