Should I declare the usage of a new template.php file?
Drupal 8 themes don't have template.php files. Now they use a mytheme.theme file, which needs to be located inside the directory containing the theme. (Replace mytheme with the machine name used by the theme.)
Preprocessing and modifying attributes in a .theme file shows an example of preprocess function, which is placed in the mytheme.theme file, instead of the template.php file.