Override theme - want to remove


After the D7-8 migration by the external company, I was left with a duplicate of the CERN theme but set as an override theme.

I now want to remove the override theme as it seems a bit odd to have an apparent duplicate overriding everything. However… I don’t know what is different in this override versus the default CERN theme.

Is there any way to find out what has changed? There may be some things in the css I want to keep (or indeed are maybe no longer necessary), but I’d like to have those bits listed only, rather than a complete carbon copy + custom bits of the default css (which may be overriding centralised changes).

I’d prefer not to simply 'switch to the default theme as the default" as my site is very big and a dipstick check is unreliable. Problems tend to be hidden for quite a time before discovery.

Website is https://information-technology.web.cern.ch/

Thanks :slight_smile:

Hi Cath
I’m dipping my toe into the Drupal forum, as web team resources are scarce today due to absences.
Your first option would be to be patient for a response.
A second option might be for you to create a test website and try out what happens when you switch to the default.
I hope that helps

Hi Cath

Thanks for your patience.

In general, it is always a good idea to remove what is no longer used or is otherwise redundant from your website(s). This includes both themes and modules. However, as you have had an external company complete the original Drupal 7 to Drupal 8 migration, chances are that you will find differences between the current, Drupal 9 ready CERN Override Theme, and the custom one you have installed. How major these differences are, if any, is very hard to say.

While it is straightforward to compare two .css files (e.g. by using https://www.diffchecker.com/) to identify and understand any differences, you are not immediately able to do a comparison between only the lines actually used by your website. This means that you could have hundreds of lines appearing different without any actual impact on your website.

Please note that several IDEs (including https://www.jetbrains.com/webstorm/ and https://code.visualstudio.com/) do allow you to identify which CSS classes, for instance, are actually used and where. You can check this by opening your website’s folder and browsing to the different .css files after the IDE has finished its initial scan. However, this requires you to be comfortable working in such environments. If you are not, no worries!

In order to compare a .css file from your custom installation to the equivalent in the CERN Override Theme, start by heading to https://www.diffchecker.com/. Once loaded, open and copy the .css file from your local installation. Paste this in one of the two boxes. Once pasted, find and copy the relevant .css file from the CERN Override Theme (see https://gitlab.cern.ch/web-team/drupal/public/d8/themes/cernoverride). The file(s) will likely reside in the same (sub)folder(s). Paste this into the other box and click Find Difference.

If the original contract had specific requirements, chances are that the external company only delivered exactly what you requested, and nothing more. There is thus a chance this corresponds to what you are actually using today. Whilst I do not have access to your website, it does not immediately appear to use any substantial amount of custom logic.

I recommend proceeding with Kate’s second suggestion: Create a test website and see what happens.

Let us know how it goes!

All the best!

Thanks for such a comprehensive reply!

I’ll look into this after the D9 upgrade. Thank you so much for all the pointers.


Hi Cath

Sounds good, please let us know if there is anything else!