Issue Type: Feature Request

VS Code version: Code 1.23.0 (7c7da59, 2018-05-03T16:44:55.614Z)
OS version: Windows_NT x64 10.0.16299

My extension also renders active indents, and I want an easy way to disable the inbuilt ones, rather then tell users to generate their own theme and then copy the editorIndentGuide.background value