In Microsoft HTML Help CHM files only the actual topic pages can be "styled" by Help+Manual. The CHM Viewer itself is a standard Windows component that is only configurable to a very small extent.
The configurable features of the help viewer are controlled by the help window definitions in Configuration > Microsoft HTML Help > Help Windows in the Project Explorer.
•In HTML Help the help window settings define the appearance and functions of the help viewer, including the controls displayed in the viewer, the text displayed in the viewer title bar and how secondary and external windows are handled.
•The help window settings also define the initial size and position of the HTML Help viewer and secondary windows displayed as external windows (if you are using them).
By default only one default help window type called Main is defined. This is the standard window type used for all topics. The settings for Main control the appearance and behavior of the main help viewer in HTML Help.
Windows other than Main are only used to open topics in external windows when you link to them. This is done by specifying the secondary window type in the Window: field in the Insert Hyperlink dialog.
Help Windows (Project Configuration)
Help Windows (Reference)