i am trying to customise the mega menu widget to change the colour scheme, however i cannot seem to locate the actual css file that relates to this, kindly advise further if possible
If you are making customizations, I recommend adding your custom css to override the existing css into a dedicated style sheet using a child theme, or adding it to the Appearance > Theme Options > Custom CSS panel.
The core/base styles for the mega menu are coming from the Max Mega Menu plugin. From there, the theme is modifying to match the theme out of the box in two primary places: (1) the polytechnic/theme-core/theme-assets/stylesheets directory and (2) dynamically in polytechnic/theme-core/ot-user-styles.php.
Again, if you are planning on making customizations to the core theme files, I highly recommend a child theme so that you don't loose your changes when you update the theme. The theme-download-package includes a starter child theme to help out. Customizations like this, though, reach beyond the scope of support by nature.
Let me know if that helps and if you have any other questions.