Hi guys!
How you solve this "problem": using dark theme (Tomorrow Night in my case) with very light Preview:
Maybe I need to use some .css? But I don't know where to find this .css 
In the beta release of RStudio 1.1, this wasn't a "problem". The viewer pane matched the theme.
I don't see this as an issue though and that's why I put the word problem in quotations. I use blogdown a lot so I want the viewer pane to show me the true output of my .Rmd document. I don't want it to match my editor theme.
We decided to avoid manipulating the CSS used for content displayed in the Viewer pane, as there wasn't a way for us to consistently manipulate arbitrary HTML content such that it displayed nicely in a light vs. a dark theme.
You'll have to explicitly give your R Markdown document a theme, using the instructions at http://rmarkdown.rstudio.com/html_document_format.html#appearance_and_style.