I modified it in the theme. See it on my test-site : https://blackland.legtux.org/

For instance, in Bootstrap Darkroom, on line 12 of footer.tpl, I find :

{'Powered by'|@translate}  <a href="{$PHPWG_URL}" class="Piwigo">Piwigo</a>

I modified it so the link opens in a new page or new tab of the browser, like that :

{'Powered by'|@translate}  <a href="{$PHPWG_URL}" class="Piwigo" target="_blank">Piwigo</a>

Best way is to use the template extension system so that the modification will not been lost with next update of the theme.

Try it in themes/default/template/footer.tpl
When you can(t find something in your template, it's because it's using the default template.
I did not try this.

With Piwigo, there's a technique for modifying templates as you wish, while preserving your changes when updating templates. Beware, the use of Local Files Editor is for advanced users. See here the documentation :: https://doc.piwigo.org/self-hosting-piw … tor-plugin

1/ First, you have to install the Plugin Local Files Editor. Maybe you have it already.

2/ In the 1st tab, you will enable the template extension link so that it appears in the side menu under /Configuration
Enter following code and save it.

$conf['show_template_in_side_menu'] = true;

Now you should see Template in the side menu.

3/ Back to the plugin Local file Editor, template tab.
Create a new file and give it a personnal name and choose the model from the scroll menu. In your case : Themes/modus/template/footer.tpl, the one you modified, though personnaly, I was not able to find this file.
Modify this template file as you wish (as you did with your file)  and save it. It will be saved in the /local folder.

4/Click in the sidebar menu on Templates under Configuration.
Choose your modified file with the new name in the 1st column and on the same line, last column, choose the file it will replace. Then save the page.