Core Service client, by default uses currently logged on user unless you want to use a different user, like it is explained here in the SDL docs:
You may probably have to impersonate it, for example:
I kindly received the following details from one of our developers who explained you can modify the list data, filtering logic, and styling in the Activities screen. You also have an extensible area to change or add buttons above the list in case you want to change create additional "subtabs" for the activities.
The list/columns are ...
What I ended up doing a long time ago when I had the same need was to have a different extension for each command, then:
Set a variable with the name of the command the user called (this.properties.CommandCalled="Save") or something along those lines
Call a "central" function which had the logic I wanted to execute
In that &...
If you're willing to use the environment name for that message, you can follow the instructions to skin the Content Manager Explorer for SDL Web 8.5.
I created an older example, but for SDL Tridion 2011 on Tridion Developer. The main part was to uncomment and update the environment > name node in CME.config: