Removing frontend-web-css references from your custom code, if there is any, would be the first step. Although since Liferay 7.0 there have been numerous changes in the code and if you use any styling in your javascript override module, it could easily break when implementing it in Liferay 7.4.
There are a few components infrontend-js-webwith style overrides, and they are overriding the latest styles, either from the theme or from Clay. The removal offrontend-css-webdid affect styles all over DXP. The developer team has confirmed that in the out of the box Liferay they fixed all the classes/styles/markup affected by the above changes.
All the styles used in frontend-css-web are being replaced with other classes or moved to other modules (frontend-theme-styled) – if your module, for example frontend-js-web is using a CSS class that was used in the frontend-css-web module, removing it should be ok, since the styles do not rely on those classes anymore.
