I love RStudio, but find quite annoying a very easy to improve feature: when you double click on some word in your code, it automatically highlights all the ocurrences of the same word in the code, but it draws a difficult to distinguish soft rectangle around the word. I think just highlighting the background colour on those words makes them far easier to find at a glance. An example:
You can change the appearance of RStudio to suit your needs.
If you go to Tools > Global Options > Appearance, you can choose different themes, which can effect how RStudio highlight pieces of code.
These are only the default themes included with Rstudio. If you want to know more about the themes, you can see the blog post here
Linked within that post is the tmTheme editor, which can allow you to create your own RStudio theme, which can be imported through the Global Options > Appearance menu that I described above. You could probably have some fun in there creating your own personalised theme to tweak RStudio exactly how you want it