[LyX/master] Introduce default box frame color (#12921)

[José Matos]

On Sat, 2023-09-30 at 08:38 +0200, Juergen Spitzmueller wrote:
> 
>     Introduce default box frame color (#12921)
>     
>     This better aligns with dark mode

Hi Jürgen,
  what I am about to report is unrelated to this file format change. I
have just noticed it because of it.

When opening a file I got a warning:

.../lib/lyx2lyx/lyx_2_4.py:4714:
SyntaxWarning: invalid escape sequence '\h'
  cmd = "\href{" + target + "}{" + name + "}"

the problem is the \h.

The two possible solutions are:

to change "\href{" to either r"\href{" or "\\href{".

I prefer the former because the code becomes easier to read but any
option will fix the warning.

FWIW the code works because there is no \h escape sequence but, in any
case, it is better to fix this.

Best regards,
-- 
José Abílio