There are two panes for the custom CSS code.
The code from the CSS pane is added to the end of all other CSS code that Sparky Framework generates automatically, according to the values in many parameters. So, the CSS code entered here will override automatically generated CSS code.
In example, if you set the body background color to black in Body Background sub-tab of the Style tab, and enter custom CSS code that makes the body background white, your custom code will override value from parameters and your site’s body background color will be white.
Similarly, you can override all other settings or code additional CSS properties of the HTML elements. In example, if want to add background color or border to the headings, you can’t do that from there parameters (there’re no such parameters). However, you can write your custom CSS code here.
The code from the “CSS (RTL)” pane is applied only when the “Right-to-left” option is selected for “Text Direction” parameter in the Layout tab. So, this CSS code is only for the right-to-left (RTL) languages.
You can also write jQuery code here because this library is loaded by the Sparky Framework. You can also write jQuery UI code, but make sure you enabled it in the Scripts sub-tab of the Features tab.