Hi dani,
it's strange, i tried the same change in template.less in my theme folder and works for me.
maybe a cache problem?

I have thought about if it can be a cache problem. But which cache? I have emptied the browser's cache. I have clicked Purge Expired Cache in Joomla backend and cache folder is empty. But it wouldn't really make sense, because the physical file in ftp should be updated even if it was a cache problem.

If I rename css/themes/mytheme/template.css to somthing else, compile less to css, then a new template.css file appears, but with the same old values, not the new values from template.less. The file in the assets folder, which is the file the browser use, is not re-created.

if you are in development mode, the changes to t3_blank/less/themes/yourtheme/template.less file should be visible immediately, without compiling less to css.
In dev mode t3_blank runs less files.
Are you working on a custom theme or on a existing T3 theme (green, dark etc...)?
Is your website online?

I am working on a custom theme.

Link

i tried to make a changes to your template with firebug, i changed the color of footer text in the style.less file, and it works.
Are you using the "scream" theme?