When subscribing to a Microsoft Office 365 P1 plan the website that gets created for public facing does not directly allow custom branding. For this plan Microsoft does not enable publishing feature as well.
The master pages found in the masterpage library are not used for the public website. The default master page is called root.master and can be placed in the root of SharePoint. Click ‘All Files’ in Sharepoint Designer 2010 to see the file hierarchy of SharePoint and you will find the Root.master at the bottom of this listing.
We cannot modify root.Master, instead a new master page could be uploaded and set as default master page.
You could try the following to test this out:
1. Use a starter master page found at: http://startermasterpages.codeplex.com/
2. Use SPD to upload the new master to “_catalogs/masterpage”. Then right click the new master page you just uploaded and “Set as Default Master Page”. Then inside each of the pages “pages (web Pages)” you will need to edit the “MasterPageFile="~site/root.master"” to be something like “MasterPageFile="~site/_catalogs/masterpage/myCustomMasterPage.master"”. This will allow your site to use the new master page.
One additional thing is you will need to add the content placeholder office 365 pages use. It’s not the same as the standard SharePoint master page content place holder.
Add the following to just above or below the SharePoint content place holder (PlaceHolderMain) in your new master page:
<div id="IWS_WH_Elem_Content" class="MSC_Body"> <asp:ContentPlaceHolder ID="IWS_WH_CPH_Content" runat="server"></asp:ContentPlaceHolder> </div>
A site like below could be easily created on a P1 plan: