[Calzifer]

> I understand that search bar position is not changeable by theming,

It is changeable. With enough dedication you can go a long way just with CSS.

In this case it is even rather easy because the "unified toolbar" the thing containing the search box, the menu bar (if shown) and the tab bar are three elements in the same flex box. They can be reordered by setting the order property.

Only downside in this case is that (if client side decoration is not disabled in the settings) the window buttons (close, minimize) are also part of the unified toolbar and would end (without further fixes) below the tab bar.

As a quick (and dirty) experiment I moved the tab bar left to the search bar in the same row just with:

  #titlebar {
    flex-direction: row;
    > unified-toolbar { order: 2; width: 50vw; }
    #tabs-toolbar { order: 1; width: 50vw; }
  }

And a hacky way which often works good enough is to reposition and hardcode stuff with position:absolute/fixed/sticky.

Finally Thunderbird's own customization dialog can be used to fill the empty space around the search bar. By default it has a spacer left and right but that is easy to change even without custom CSS.