How to customize Jira

Grin_E

I want to close the task of customizing Jira.
The look and feel functionality is not enough for me. Ideally, I want to be able to change the Jira background and the HTML markup.
Maybe someone has experienced it or at least has an idea
I would be glad to get any tips

Grin_E

As far as I understand, the page is formed by HTML, then we need to look for options to change it. We need to find a way to change the page and have this script work every time you visit the site.