Adding the Custom Css classes in Rich Text Editor under the Apply Css Classes dropdown

Adding the Custom Css classes in Rich text Editor need to be done in below two steps.

1) Add the your custom css file in under the Inetpub/wwwroot/[Your website]/YourCssFile.css
2) Add this file path in Sitecore.config under the [Yourwebsite]/Appconfig/Sitecore.config

after the default.css setting like below

<setting name="WebStyleSheet" value="/default.css"/>
<setting name="WebStyleSheet" value="/YourCssFile.css"/>

Classes under YourCSSFile.css:

Check Your Classes are coming on the Rich Text Editor Full under the Apply Css Class Dropdown.
If you unable to see the your classes in Apply Css class dropdown in Rich text editor Full:

Add your classes in the Tools.xml file in Sitecore in below path
C:\inetpub\wwwroot\[Your Site folder]\Website\sitecore\shell\Controls\Rich Text Editor
under the <classes></classes> in Tools.xml File as below

RTE: