5
Close, Mestophales, but not quite there.
The file in templates_c directory is a compiled version of the theme's theme.html and as long as it is there, it will be used - however, it should not be edited manually.
Instead you should modify the theme.html in the themes folder and enable the "Update module templates from themes/yourtheme folder" in system admin -> preferences -> general settings as this will recompile the theme.html on every page load. When the theme is as you want it, disable the setting in system admin again for improved performance.