Drupal 8 block css add
The CSS changes are applied through a subtheme, but if you don't have access to that or only need to make small adjustment you can use the CSS Editor module to add your CSS. 

The HTML you supplied doesn't seem to have a unique attribute you can use to target it reliably. Is there are wrapper div with an ID attribute around that code? 

With classes you will likely style more than one component, this selector targets elements with both these classes: 

.field--name-body.field--type-text-with-summary {
 float: right; 
}


BTW you need to test your code in different browsers and browser window sizes to make sure you're not breaking the layout in some cases.