Theme creation - front page with prepopulate data (like hero block)

@Dominique - Actually, I think a little more information might be necessary to help answer your question. 

* Say more about "The idea is to put some images that could be easily modified by everyone". Who is "everyone" and do you want everyone to modify these blocks?

Upon reflection, I am thinking that you mean any site owner or site architect that uses your theme on their site?

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One of the big differences between BackdropCMS and Drupal is that the placement of blocks on the front page is handled by layouts and not themes. In Drupal 7 world, a theme might include templates that specifically handle the placement of blocks. In Backdrop, you should really be using layouts for this. The theme is much more of a skin that you can apply to layouts. The theme is more focused on mostly the CSS. 

Having said that, some use cases for blocks are not handled very well by existing contrib layouts and themes. Sometimes, a theme might need to modify existing core layouts or provide support for a specific usage of blocks. 

I've been working on the Opera theme for one such situation that might be similar to what you are trying to do. I recommend taking a look at Opera and how it's structured, it might give you ideas for your own themes or you might have ideas on how to make Opera better. I welcome ideas/collaboration on Opera. 

https://backdropcms.org/project/opera

Maybe my question is : is it possible to avec a theme that add hero blocks or blocks everywhere in the page (top, left side, right side, footer...).

(By default the hero block is on top region) : i want more than one and not only on top region.

@Dominique - I think I am closer to understanding what you want. But, I might not be there yet. :-) Don't worry about the English, we'll figure it out. 

By default, everything in Backdrop is a Block - which means that of course you can have blocks everywhere. This is also the difficulty in creating a "sexy" theme by default, because any kind of block can go anywhere - so it's hard for a theme to account for all possibilities. 

I agree that only having the ability to put hero blocks in the top region is a problem for Backdrop, which is why I created Opera theme. The Opera theme also allows for hero blocks in the content region (but only on the front page). 

It is really the layout the determines where you can put hero blocks. The Opera theme makes adjustments to ALL core layouts to allow hero blocks in the content region, but only on the front page. You could make the same adjustment for other pages as well if you like. 

One approach for you, might be to create a special layout for your theme and recommend that users using your theme also use that layout. You could release that layout as a it's own contrib project OR, you could include it in your theme (but that's not how Backdrop CMS is "supposed" to work). 

I hope that these comments are helpful. I would love to talk about this with you, since I think I have some similar ideas with Opera. BUT, I think you might be looking for something more specific. 

The flexibility that layouts provides in Backdrop CMS also makes it harder to create very specific contrib themes with specific layouts or functionality.