feat(bslib_page): Set global theme temporarily during render

[gadenbuie]

This is a draft proposal that fixes #813. In essence, it attaches the page-level theme to the bslib_page classed object in the "bs_theme" attribute. I then modified print.bslib_page() and as.tags.bslib_page() (not sure if that's necessary) to temporarily set the global theme using this attribute.

In #813 the slider in the following example is only partly styled, with this fix bs_dependency_defer() correctly uses the current theme.

library(crosstalk)
library(leaflet)

# Use crosstalk to create a client-side filter between the map and slider
quake_dat <- SharedData$new(quakes)
map_filter <- filter_slider("mag", "Magnitude", quake_dat, ~mag)
map_quakes <- leaflet(quake_dat) |> addTiles() |> addCircleMarkers()

page_sidebar(
  title = "Client-side filtering",
  sidebar = map_filter,
  # Can also put other bslib components here
  # like cards, value boxes, etc.
  map_quakes
)

Before	After

[cpsievert]

@gadenbuie thanks for this, I like the simplicity/usefulness/correctness tradeoff.

This does, however, sort of make #477 a more universal problem. I'm thinking we'll address that by making htmltools::save_html a S3 method (in a follow up PR)?

[cpsievert]

Sorry to merge before giving you final approval, but this'll make the follow up PR easier to get setup