My 2 cents. (and
for not starting out saying what is there is
terrible )
The wonderful thing about Calibre is it
offers different UI's and customizable toolbars. AND IT RUNS ON 3 OS
I run with Multi 1920x1080 monitors. (some folk run on a Tablet)
I rarely use grid view' I hide columns of little immediate interest. Those that are there, are reordered.
I tend to keep the Tag Browser and Book Details Closed (I frequently leave the Info pane on another monitor. It has been customized). I want wide details (my book details pane, when open, is at the bottom).
I use ALL 3 toolbars (Menu: Preference, Help. Main: Standard Calibre + Device related >, Second Toolbar: various tools (mostly 3rd party)
We already have the ability to have Icon Themes (we already had the ability to put any custom replacement Icon into the Preferences folder: Resources from a very early days)
Maybe you can come up with a way for more of the UI to be customized BY THE USER
who wants a totally different look and feel from the base Calibre.
Kovid has the final say as to what is BUILT IN, but plugins are open season
Go for it