[css][xs]: remove curved borders from grid headings.
[css][xs]: remove curved borders from grid headings.

maxogden/recline,acouch/recline,okfn/recline,gotjoshua/recline,huangciyin/reclinejs-requirejs,okfn/recline,modulexcite/recline,beni55/recline,NuCivic/recline.js,maxogden/recline,NuCivic/recline.js,NuCivic/recline.js,acouch/recline,modulexcite/recline,GerHobbelt/recline,GerHobbelt/recline,rjsteinert/recline,rjsteinert/recline,modulexcite/recline,boban-dj/recline,boban-dj/recline,okfn/recline,acouch/recline,gotjoshua/recline,beni55/recline,gotjoshua/recline,boban-dj/recline,huangciyin/reclinejs-requirejs,GerHobbelt/recline,beni55/recline,rjsteinert/recline