#include "TROOT.h"
#include "TStyle.h"
#include "TH1.h"
#include "TCanvas.h"
void test () {
gROOT->ForceStyle(kTRUE);
gStyle->SetHistLineWidth(6);
TH1D *h = new TH1D("h", "", 100, 0, 100);
for (int i = 0; i < 100; i++) { h->SetBinContent(i+1, i); }
TCanvas *c = new TCanvas("c");
h->Draw();
c->Modified(); c->Update();
} See also: Changing style of 2-D histogram