Regarding the style and visual design of ZX Opal:
The terminal and shell: I'm leaning on making white on blue the default, which historically matches quite a lot of precedent, from Atari BASIC to Locomotive Software's CP/M to QBasic to Turbo Pascal. May introduce some ANSI colour, mainly to differentiate input text from output, and possibly also stderr.
Some of the icons like the "Document" and the Rubbish bin/Trashcan icon have been around since the Xerox Alto days (pictured below).
The window title buttons are Linux/Windows-like, and the default mouse pointer is Mac-like.
The default font is TomThumb, a slight improvement on the X11 3x6 font.
There will be build options available to change the visual style options that are not easy to make dynamically configurable
for memory savings reasons.
In fact the current build system is already producing 156 different combinations of memory models and Spectrum binary formats.
For this reason, a future website (yet to be built) will let you choose from a variety of options, and let you build a ZX Opal tailored to your own preferences.
@haitchfive The icons seem from ViewPoint on the Xerox Star rather than the Alto.