Where is the custom CSS option? With all the questions regarding the preview pane, I am now looking at doing it myself. I am on the dark theme and with five minutes of time came up with the following that does work. 

I know this would need to be done for each theme but the ability is thereby us if we have the custom CSS option. This is far from ideal but I am willing to do this for my customers that demand it rather than have them leave and lose money because of it.

body.theme-dark .emailMessage, #mailContentView .mail-message-view, #mailOverview .mailOverviewArea #mailContentView #mailContentViewMessage{display:none;overflow:hidden;}

#mailOverview .mailOverviewArea #mail-info-view #mailListView.preview-right{width:100%;}