(How) may I customise the font/color/size/placement of the various notifications bubbles

[Sébastien]

Hello!

One of my user is visually impaired and after much research on why
her SD card (that she promises she safely removes) keeps getting
corrupted, I noticed that she believes every notification sent by
Thunar tells her it's safe to remove the card.

Some notifications also say that data still needs to be written and
suggest her to wait, however she can't make a difference between the
two.

I thought of colouring the background of notifications in a shade of
orange and green respectively when it says data needs to be written
on the removable storage and when it's safe to remove it.


How may I perform such task (if possible without recompiling)? Is it
feasible to stylise them?
Or perhaps only enabling the one that says it's safe to remove it,
even though I'd still like to stylise them (even for myself)?
As a last resort, can she use a different notification system that
allows more customisation?

She's using an old Xubuntu 15.04 installation with xfce 4.12, although a
17.10 upgrade is urgently planned.

Thanks,

-- 
Sébastien