You are not being very specific about what you want to change and how, nor on how the content is created.
If you are creating the content using HTML, you can just add the styling inline with the HTML like
<p style=”color:red;font-size:110%;”> …… </p>
Or you can add your own classes to the HTML like
<p class=”myclass1″> …… </p>
Then define the styling for the class by placing a CSS rule like below in Main options > Fonts & Custom > Custom CSS Rule box. (or Customize > Custom CSS > Global CSS Rule box)
.myclass1 {color:red;font-size:110%;}
Now if the content is generated by a plugin or widget, then you need to inspect the page with your browser developer tool, and check what CSS selector can be used to create a CSS rule that will style your element.
if you need help with that, be very specific about what element you wish to change and how