Hi sinhoboc
These classes are defined in a custom HTML module called 'Explore Our Trek Packages' (this module name on our demo site) in below_columns_1 position
You can go to Global Configuration and set the 'Default editor' to 'None' first so you can see these classes.
The CSS is defined in this file: /templates/outdoor_life/css/editor.css
If you want to override, pls put your custom css into this file: /templates/outdoor_life/css/custom.css (Create this file if it doesn't exist).
Regards