Why does the Mixed Content error occur when redirecting from internal pages to the main page?

Question

htaccess

Hello! I set up a rule in htaccess for redirects from all internal pages to the main one:

RewriteEngine on
RewriteCond %{REQUEST_URI} !^/$ [NC]
RewriteRule (.*) / [R=301,L]

The rule works, but now I get the following error in the browser: Mixed Content: The page at ' https://site.ru/ ' was loaded over HTTPS, but requested an insecure stylesheet ' site.ru '. This request has been blocked; the content must be served over HTTPS.
Accordingly, although the main page is loaded, not a single file is loaded, neither styles, nor pictures, nor js. The site has an SSL certificate.
As I understand it, you need to somehow set up redirects for all pages, except for certain files with an extension?
Please help me write this rule.