The custom CSS code will be appended to all other CSS settings, so that it's even possible to override design or system specified values.
If custom CSS code is disturbing in edit mode (while editing page contents), it's possible to make custom classes dependent of the view mode.
The body tag get's the class "view-edit" when in edit-mode, and "view-normal" otherwise (in preview and LIVE).
As an example, if there is a class "my-class" and this one should not apply in edit mode, just prepend it with the "view-normal" class, separated by one space.
Here on line 1 the H2 title tag inside of an element with the class "header" is addressed. Line 2 sets the line-height. This rule will always apply.
On line 5 howevery, the class "view-normal" is preceded, so that line 6, wich sets the background-color to green, will not apply when editing a page.