One approach for integrating a KTH-like style into the R web frameworks used is to generate a bootswatch-like CSS for KTH and then tell the R web frameworks to use that CSS. It needs to use Bootstrap 3.4+ but not Bootstrap 4 styling.
This can be accomplished by cloning the https://github.com/thomaspark/bootswatch.git - and checking out the “v3” branch. A set of LESS-files are then needed.
A repository is available at https://github.com/KTH-Library/bootswatch-kth which does this customizations and provides the resulting CSS files. Use the v3 bootstrap.css in the flexdashboard with the parameter “css”.
Further, there is https://rmarkdown.rstudio.com/flexdashboard/using.html#css_styles which describes that bootswatch css can be used but in addition to flexdashboard-specifics here: https://github.com/rstudio/flexdashboard/tree/master/inst/rmarkdown/templates/flex_dashboard/resources