Note: Custom code fully works on the Professional or Custom plans with the Code Injection feature. On Free and Creator, you can test this code in the Preview mode but it will be ignored after publishing.
In order to change the text selection color, create a Code widget. Double-click on it and insert the following code into its CSS section (removing all the rest):
::selection {
background-color: cyan;
}
Now you can create a text widget and select some text to see the results. Change cyan to red, orange, green, etc to see the changes.