Howdy, @mickshah! Thanks for writing in, we’ll be happy to help get you moving in the direction you’re looking to go. 
As for the blog design with the Stack you’re currently using (Ethos), the default selection should be “All” by default. When I load up your page, the dropdown says “Filter by Topic,” which in that state is set to “All” behind the scenes, just showing your most recent posts. If a user then selects a category, the posts will be filtered by that category, and the “All” selection is really just a way to get back to your “Recent Posts” in the order they’re being published. That’s what I’m seeing on your site when I check it out live and that is the expected behavior. Let me know if you need any further clarity.
As for changing up the design of your blog, since you’re using Pro, definitely check out some of the new features available in the latest version of the theme (v4.0), in particular our new Layout Builder. You can read in great detail about this latest release cycle and all of the features included here:
The Layout Builder allows users to create completely custom archive and single template designs from the ground up using the Elements found in the Builders. We give a very short overview of how to leverage this in a couple videos on our YouTube channel:
As you can see from those videos, we even have a couple really cool assets in our Design Cloud that you can download and start with that have a very fresh, modern look:
https://demo.theme.co/designcloud/layouts/postmodern-pixels-archive/
https://demo.theme.co/designcloud/layouts/postmodern-pixels-single/
You can find these assets in Design Cloud by going to Design Cloud in the navigation of the tool and searching for “Postmodern Pixels,” which will reveal the following assets for you to download if you want to start there:
Once you have downloaded those assets, you can follow the steps outlined in the videos above. We also have extensive documentation on creating and assigning your first layouts in our online docs:
That should give you a really strong foundation to start with and begin exploring all new possibilities with Pro!