Hi Huy,
Unfortunately, for some items, it's not possible to override the default system style (it may also depend on the OS). For example, on Windows 10, if you specify a background color for Pages of a Folder....
<Style name="Page">
<StyleAttribute name="backgroundColor" value="yellow" />
</Style>
....the color will be applied inside the Page but not on the header (because the Qt library we're using doesn't allow it). It's the same for buttons : if you apply a background-color, you'll only see a small border of the color you applied around the button. The same case can be also observed for Topmenus, Toolbars or for Group titles....unfortunately we cannot make all elements fully customizable if we want to stay in concordance with the system style.
Best Regards,
Lionel