* Custom EMACS color theme

Color themes are a bunch of font-face definition, generally matching a
particular color palette.

** Initial setup

You need to have a place where all your custom themes will
live. Create a folder, then add that path to the
=custom-theme-load-path= variable like so:

#+BEGIN_SRC elisp
  (add-to-list 'custom-theme-load-path "/my/themes")
#+END_SRC


** How to make a theme

Start by declaring the theme with the =deftheme= macro.

Define the theme's custom faces with =custom-theme-set-faces=.

Provide the theme with =(provide 'my-theme)= to tell Emacs it can be
used.

Add an autoload section to add the custom theme file to the
=custom-theme-load-path= var.

Thus, a very (very) basic theme would be like so:

#+BEGIN_SRC elisp
  (deftheme demo "A light theme")
  (custom-theme-set-faces
   'demo
   '(default ((t (:background "#FFFFFF" :foreground "#000000")))))
  (provide 'demo)
#+END_SRC

** Custom faces

Custom faces are defined as =(name ((NOW (:key val ...))))=. The very
basic face names of the Emacs GUI to define are:

   - default
   - vertical-border
   - fringe
   - cursor
   - bold
   - italic
   - bold-italic
   - region
   - underline
   - custom-face-tag
   - custom-state

Font names to define for a useful theme for programming can be found
[[https://www.gnu.org/software/emacs/manual/html_node/elisp/Faces-for-Font-Lock.html][in the font-lock documentation]].

** Mode faces

Each mode can define their own custom faces. For example Markdown
declares custom faces for its different header levels. Whenever you
want to add a face definition, simply place your cursor on the text
and do =M-x describe-face=. This will echo the face name in the
minibuffer.