RStudio addin which grays out lines

Hello R Community!

Currently, we try to develop a R Studio addin, which enables the user to invoke a program slicer directly from the IDE (e.g., to reduce a program to only the parts necessary to create a specific plot).
As part of this addin we would like to provide a view in which the lines that are not needed for the slice(s) are grayed out. Can we do something like this directly within the RStudio editor window (i.e., without using a separate shiny frame)?
This is our first addin so any help or pointers would be greatly appreciated! We already tried injecting css as it is done within the example addins but this didn't work. If you need further context, the project is located here: GitHub - flowr-analysis/rstudio-addin-flowr, with the most important part being us trying to highlight/dim the respective lines (at the moment, we use source markers for this):

mark_slice <- function(slice_locations, path, criterion) {
  markers <- list()
  for (location in slice_locations) {
    loc <- location[[1]]
    markers[[length(markers) + 1]] <- list(
      type = "info",
      file = path,
      line = as.numeric(loc[[1]]),
      column = as.numeric(loc[[2]]),
      message = paste0("Member of slice for ", criterion, " (", loc[[1]], ":", loc[[2]], " -> ", loc[[3]], ":", loc[[4]], ")")
    )
  }
  rstudioapi::sourceMarkers("flowr-slice", markers)
  print(paste0("Highlighting ", length(markers), " tokens for slice ", criterion))
}

Thank you very much in advance!