Required color-theme file missing in Emacs 26

After upgrading my OS (Ubuntu, from 18.04 to 19.04), my favorite text editor was automatically upgraded from Emacs 25 to Emacs 26. I then encountered a familiar error message at start-up:

File is missing: Cannot open load file, No such file or directory, color-theme

The relevant portion of my configuration file, .emacs, is the
following:

(add-to-list 'custom-theme-load-path "~/.emacs.d/themes/")
(add-to-list 'load-path "/path/to/color-theme.el/file")
(require 'color-theme)
(eval-after-load "color-theme"
    '(progn
     (color-theme-initialize)
     (color-theme-subtle-hacker)))

This works fine with emacs 25 or earlier versions, under Ubuntu
18.04. Naturally, the relevant file, color-theme-subtle-hacker.el is
located in ~/.emacs.d/themes/.

I imagine that there are at least two possible solutions to this problem:

(i) There is a way to “tell” Emacs 26 where is the relevant
color-theme package. The problem could, in fact, be due to the
OS upgrade.

(ii) I could also force Ubuntu 19.04 to use Emacs 25. However, I would have
to install it manually, since every attempt at installing Emacs
currently leads to version 26.

(add-to-list 'load-path "~/.emacs.d/themes/")
(require 'color-theme)
(eval-after-load "color-theme"
    '(progn
     (color-theme-initialize)
     (color-theme-subtle-hacker)))