Я сделал это так:
try {
Method setStripEnabled = tabWidget.getClass().getDeclaredMethod(
"setStripEnabled", boolean.class);
setStripEnabled.invoke(tabWidget, true);
Method setLeftStripDrawable = tabWidget.getClass()
.getDeclaredMethod("setLeftStripDrawable", int.class);
setLeftStripDrawable.invoke(tabWidget, R.drawable.tab_line);
Method setRightStripDrawable = tabWidget.getClass()
.getDeclaredMethod("setRightStripDrawable", int.class);
setRightStripDrawable.invoke(tabWidget, R.drawable.tab_line);
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}