tdf#91501: new translatable string

l10n
#1

Hello world,

so I didn't notice I'm on a wrong git branch and pushed the fix for
https://bugs.documentfoundation.org/show_bug.cgi?id=91501 to 5.0 branch by
accident ...

As $subject already says, it sadly introduces a new translatable string
("Custom" in a sense of "custom colour, not in the default palette"). That's
the bad news. The good news is, such a string is likely already translated
elsewhere, so ...

Sorry 'bout that, I didn't want to add more work for you guys on purpose. I'll
now repent by writing at least one unit test

B.

#2

The bug fix looks okay to me -- as you said, in this case the string
is pretty common and is already translated in other places. Don't
worry too much about it =P; and I hope the commit to 5-0 does not get
reverted -- please take this message as my "sign-off".

Thank you,
Adolfo