#set text(font: "Times New Roman", size: 12pt)
#show table.header: set text(weight: "bold")   // A: show-set
#table(
  table.header([Aaa], [Bbb]),
  [ccc], [ddd],
)
#pagebreak()
#show table.header: it => { set text(weight: "bold"); it }  // B: show + set внутри
#table(
  table.header([Aaa], [Bbb]),
  [ccc], [ddd],
)