Improve margin and highlight color settings for Dracula theme.
Improve margin and highlight color settings for Dracula theme.

Related to issue #3420, where margin isn't set correctly for some dark
themes.

JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio