If you check Specification Category you will find one quite interesting thread discussing the options to "customize" EHS classic.
This option shows the "highest" option (which I know) in hierarchy you can use to start the work and generate a complete different EHS Classic "look and feel". You should only use this option if you really have a good picture about the SAP architecture used (e.g. reading, writing data etc.).
The options to change EHS classic function using the SAP standard are quite large. Some examples are mentioned here:
If you dig deeper in the technical part of this set up you will immediately identify additional options to "change" lool and feel of EHS. Some of these options are really helpful. PLEASE DO NOT CHANGE set up of standard SAP Value assignment type and if so: DOCUMENT WELL your changes.