On the left edge of the document, where the line numbers, code folding markers and breakpoints go, is there a way to set this to a different background color than the text background? I keep thinking I need to de-indent or that my code is indented more than it is.
Yes!
For linenumbers and breakpoints: Preferences - Fonts & Colors - Common Syntax, Element Type: linenumbers
For fold margins: Preferences - Fonts & Colors - Colors, Color choice: Fold margin color.
Also in KIDE9 (currently is a pre-version) Fonts & Colors replaced with Color Scheme.