This is a set of examples of how to make presentations using RStudio, Rmarkdown and in paricular Xaringan.
To install Xaringan, look here. An excellent showcase of Xaringan functionality is available here, made by the author himself.
RStudio's Alison Hill has a number of really excellent examples further showing the use and features of Xaringan:
Meet Xaringan: making slides in Rmarkdown
R-Ladies Presentation Ninja, with the code available here.
If you do really want to make your slides look amazing, it will be well worth your while delving into CSS, at least a little bit. It will give you far more control over the look of your slides. If you don't know what CSS is, @rosemckeon's introductory Coding Club session is excellent. John Little's Duke University template slide deck, is an excellent example presentation which uses custom CSS to modify slide background colours, add colour text boxes etc. Have a look at their CSS file here, specifically the .content-boxes.
I also have an example of a lecture on modelling in conservation science, made in Xaringan, here..