\switchtotypeface [Custom] [10pt]
\setupheader[style={\switchtotypeface[Custom][12pt]}]
\setupfooter[style={\switchtotypeface[Custom][12pt]}]
\switchtotypeface [Custom] [10pt]
\setupheader[style={\switchtotypeface[Custom][12pt]}]
\setupfooter[style={\switchtotypeface[Custom][12pt]}]