pkgdown: "standard" theme

[Yuri05]

Let's set the theme of the online documentation to "standard" (dark text, white background)

Also some parts of the vignettes in the online documentation are not displayed correctly. Not sure if it's because of the theme or other settings

E.g. s. https://www.open-systems-pharmacology.org/OSPSuite.ReportingEngine/articles/introduction-re-instances.html#nomenclature

[IndrajeetPatil]

Change the following in _pkdown.yml:

template:
  bootstrap: 5
  bootswatch: cyborg
  theme: arrow-dark
  bslib:
    primary: "#0054AD"
    border-radius: 0.5rem
    btn-border-radius: 0.25rem

to

template:
  bootstrap: 5

And then run:

pkgdown::build_site()

Like here.

Sorry, I can't build the website locally because there is something wrong with PK installation, so vignettes don't build on my machine.