Change blogger contempo theme to keep archive open

 

In the new templates (Contempo, Soho, Notable, Emporio and Essential), the gadget Archive works like you mentioned.

 

So to change that, please first save a backup of your theme, then:

1. Open the HTML editor of theme: Theme --> My Theme --> Edit HTML
2. Click on "jump to widget"
3. Find the Archive gadget
4. Expand its code
5. Find this line           <details class='collapsible extendable'>
6. Change that to this           <details class='collapsible extendable' open='open'>
7. Save and test.