Adjust ui_*() colors based on RStudio theme

[jennybc]

The darkgrey used by ui_code() is hard to see in some RStudio contexts. Examples where RStudio is offering debugging facilities:

When cliapp matures or another option presents itself, take advantage to make styling adapt to a dark vs. light theme, e.g. consult rstudioapi::getThemeInfo()$dark.

https://github.com/r-lib/cliapp/blob/cbf8f1a692a44b12db508a2050c9133487ae07bc/R/simple-theme.R#L71

[hadley]

@jimhester do you think this should be resolved in RStudio so that the "darkgrey" colour is still readable with a dark theme?

[hadley]

I think this is related to rstudio/rstudio#4076

[hadley]

I'm now convinced that this is really an RStudio issue, so closing in favour of rstudio/rstudio#1959