sidebar

You’ll have to set the permissions of the theme’s sidebar.php file to 666.

How do I do that?

I have changed theme permissions to 666 but nothing has changed.
Is it possible to get the sidebar edititing box as it is for the main content area?

Are to talking about editing the Sidebar template in Appearance/Editor or adding widgets at Appearance/Widgets? And what is the user role of your customer when logged in?

My customer has admin rights when he is logged in. Right now sidebar is hardcoded and it appears the same way on each page that is included in.
My customer wants to have abbility to change a text inside the sidebar the same way he can in a main content area now. I have set the permission to 666 as you have said but nothing has happened.

So, your customer cannot save changes to the Sidebar template at Appearance/Editor in the admin panel?

He can

Then I guess I don’t understand what you want.

Right now, there is only one section inside of each page that can be edited (changing text, images or else) through admin panel/pages/edit.

My customer wants me to make a sidebar the same way.
Right now it is hardcoded and the content of the sidebar is the same on each page.
He wants a different sidebar content on each page, but he wants to change it by him self, through page edit in admin panel, not through the theme editor or css.

I do not believe that such a thing is possible. The closest you could probably get would be to use widgets, but those would be static across the blog and would not change for specific posts or pages.

Sidebars are bound by the content in the Sidebar template or the widgets assigned to it. The Sidebar template is then called to display by another template. It is possible to have multiple sidebar templates and then use custom post templates to call different sidebars, but it’s hardly dynamic (you’d need a sidebar template and a custom post template for each post).

There are plugins out there that allow you to post “mini-posts,” “asides,” or “side-notes” to the sidebar, but these would also be static across the blog.

https://codex.www.ads-software.com/Customizing_Your_Sidebar

I take that back.

Theoretically, you could develop a plugin that would display custom fields as a widget in the sidebar. Once that is done, just enter the custom field with each post and the content of the field would be displayed in the sidebar alongside the post.