.rs.api.getThemeInfo() no longer returns foreground and background colors

[javierluraschi]

I believe there was a good reason to remove from .rs.api.getThemeInfo() the foreground and background fields by adding support for custom themes, but r2d3 and other packages could still make use of getting the foreground and background programmatically.

Related to rstudio/r2d3#50

[jmcphers]

If memory serves, we removed this because it was implemented by hand-extracting foreground and background colors from the hard-coded theme list. Now that you can load any old CSS file as a theme, attempting to scrape out the fore and back colors is somewhat difficult without doing some level of finicky parsing and interpretation of the CSS.

I wonder, though, if there's another way to do it -- we could have the browser send the server its current theme values (just by reading the computed style off the screen) after application and whenever the theme changes.