Okay, after a short crash course in basic CSS style formatting and extensive fiddling with Tasmota's 'xrdv_01_webserver.ino' file via Atom, I finally got what I was after initially, narrower buttons and a black background, yay!...and for fun, I also made an SFOS style launcher icon to start Tasmota.

Also, I can now pinch-to-zoom the UI - handy for tired eyes late at night!

If anyone is remotely interested in this subject, leave a comment and I'll upload my current 'firmware.bin' for use with a Tasmota converted 'Sonoff Basic Switch'.

My file size when built: 520kb

Original UI on the left, my custom UI on the right - much better, no?