Don’t use Pages! Instead, use the Link List gadget:
Define your topic
1 make sure you have used consistent tags (‘Labels’ in Blogger) in all the posts you wish to include in this topic
2 Design > Page Elements > Add a Gadget
3 select Labels, make choices and Save; the Labels Gadget should now appear in the Sidebar of your blog
Get the url for the topic
1 in the Sidebar, click on the Label you wish to use
2 Copy the resulting url from the Address Bar at the top of the window (Paste it somewhere for safe keeping)
Set up the Navigation Bar
1 Design > Page Elements > Add a Gadget
2 select Link List
3 in New Site URL: Paste the url you Copied earlier
4 in New Site Name: write the title you want to appear in your Navigation Bar
5 in Title: you can name the Navigation Bar if you wish
6 don’t forget to Save before leaving!
7 in the Design > Page Elements screen, drag your new Link List Page Element to the position below the Page Header (if it’s not already there)
8 your link(s) should now appear as Tabs below the Header; they will follow the colour scheme of the Template you are using
9 you can always go back to the Link List gadget and add further tabs; you can also Edit, Delete or change the order of existing ones