Moving from 24.5 to the most recent in git, 27.0.50

[jonetsu]

Hello,

I'm using emacs since some time although it's the one provided by the
distro that happens to run, to which I add a customized .emacs and a
color-theme.el in the proper site-list directory.

I have compile the latest from git and did a local install
(/usr/local/) and did not remove the previous since the local one will
have precedence according to the PATH definition.  And indeed, the
brand new one is called.

But there are two problems so far.  I have put the color-theme.el file
in its site-lisp directory in :

/usr/local/share/emacs/27.0.50/site-lisp/

And do not know if I should move my current .emacs from the home
~/.emacs directory - I think not because when emacs 27 starts for a
very brief moment the colors I'm used to flashes by and then it's all
white.  Moreover, trying to use Tools -> Color Themes yields a totally
empty white screen where the cursor cannot move, eg. truly empty.

How to fix that and have my original .emacs file take effect ?  And,
should I continue with 27, or should I perhaps go with what would be a
stable version eg. one of the current 26 series ?

Thanks for all info., much appreciated, hoping to get back on track
soon.

Ah, I did the upgrade basically because I would like to benefit from
any new feature that might help in supporting using Babel in org as
well as the Plant UML interface.

Cheers.