See the style schemes introduction and the list of incompatible changes to the file format.
This page describes how to create or modify style scheme files.
A style scheme file is written in XML.
  The file needs to have the .xml extension. For example:
  classic.xml
The first line needs to be:
<?xml version="1.0" encoding="UTF-8"?>
  A style scheme file has only one top-level element:
  <style-scheme>
Example:
<style-scheme id="classic" _name="Classic" kind="light"> <!-- ... --> </style-scheme>
  The <style-scheme> tag supports the following attributes:
id (mandatory)id. During the loading,
    if two or more style schemes have the same id, only one is kept
    (the one with the highest priority, depending on the order in which the
    files are loaded).
  name (mandatory)_namename attribute, except that it will be
    translated. name and _name may not be used
    simultaneously.
  kind (mandatory)
  The style-scheme element may contain the following elements:
  description, _description, color and
  style.
  Description of the style scheme. The <description> value is
  not translated, while <_description> is. It is an optional
  tag. If present, only one description must be set. Example:
<_description>Classic color scheme</_description>
  When a color value is needed inside a <color> tag, it needs
  to be a "final" color value (see below).
  When a color value is needed inside a <style> tag, it can
  either be a "final" color value, or a reference to a
  <color> tag.
  All "final" color values must begin with #. Examples:
#000000
  #rgba(255,0,0,0.5)
  #beige
  
  See the documentation of the gdk_rgba_parse() function (part of
  the GDK library) for more information. Note that the # must be
  prefixed if it's not already the case.
  We refer to a <color> tag by its name. Example:
<color name="aluminium1" value="#eeeeec"/> <style name="line-numbers" background="aluminium1"/>
  The order is important: a <color> tag must be located
  before any reference to it (a “define-before-use” rule).
  The <color> tags define color names to be used in
  <style> tags. It has two attributes: name and
  value. value is a "final" color value as described
  above. Example:
<color name="aluminium1" value="#eeeeec"/> <color name="with-transparency" value="#rgba(255,0,0,0.5)"/>
  Note that <color> tags are optional because a
  <style> tag can have final color values too.
  <color> tags permit to avoid duplication of values, and to
  give them names (like a color palette).
  Each <style> tag describes a single element of style scheme
  (it corresponds to a GtkSourceStyle object in the library API).
  A <style> tag supports the following attributes:
name (mandatory)foregroundbackgrounditalicboldunderlinePangoUnderline ("none",
    "single", "double", "low" or "error"). GtkSourceView versions <= 3.16
    only supported "true" or "false" and such values are still accepted for
    backward compatibility.
  underline-colorstrikethroughscale"1.75") or one of the values
    "xx-small", "x-small", "small",
    "medium", "large", "x-large",
    "xx-large". Useful for example to highlight titles or
    sub-titles (headings).
  use-styleuse-style attribute is used, the other attributes must
    not be used at the exception of the name. The value of
    use-style is the name of a previously-defined
    <style> tag inside the same style scheme file, to re-use
    the same style. So the order in which the <style> tags
    appear is important in this case (a “define-before-use” rule).
  
  The name attribute of a <style> tag can have
  special values, which control the general appearance:
textselectionselection-unfocusedcursorforeground attribute is used for
    this style.
  secondary-cursorforeground attribute is used for this style. If this is not set
    while "cursor" is, then a color between text background and cursor colors is
    chosen, so it is enough to use "cursor" style only.
  current-linebackground attribute is used.
  line-numberscurrent-line-numberbackground attribute is used.
  bracket-matchbracket-mismatchright-marginforeground attribute is
    used for drawing the vertical line. And the background
    attribute is used for the background on the right side of the margin. An
    alpha channel is applied to the two colors. For a light style scheme, a good
    color for both attributes is black. For a dark style scheme, white is a good
    choice.
  draw-spacesforeground attribute is used.