There are times when a theme's layout doesn't meet the needs of your page or post, such as when an image or text box would look better shifted to the left or the right. As of WordPress 4.7, you can modify the theme’s CSS in WordPress as follows.
Note: Making CSS changes is tied to the specific theme you are using. If you change your theme, your custom CSS changes will no longer be active.
Note: These changes take effect immediately in the Customize interface once they are entered.
For detailed information on WordPress generated CSS classes, review the WordPress.org Codex: CSS.