I am preparing to change my forum to https, but of course when I check the source code in the browser inspector, I can see that theme element images are referred as http. What will happen to those links in case I change to https? Do I need to rewrite everything manually? If yes, which is the file that generates those links in the final forum page displayed? The images I mean: the background, icons, etc. so the graphic elements of the theme.
Check the URL settings of your theme: Administration Center > Themes and Layout > Theme Settings.
When you switch to https, you also need to switch ALL of the SMF associated URLs.
Use repair_settings.php
Since the OP has been here yesterday with no further input on this issue, marking solved.