Use custom pygments styles with better color contrast in the docs

We use the same styles as before (sphinx for light mode and the Furo default
of native for dark mode), but they have been updated so that any text colors
that were lower than 4.5 color contrast have been modified. The styles mostly
look the same but some parts may be easier to read. The 4.5 color contrast
makes the docs WCAG AA compliant.

doc/src/_pygments/styles.py

@@ -0,0 +1,102 @@
+"""
+Sphinx styles used for theme.
+
+These are based on the Sphinx style (see
+https://github.com/sphinx-doc/sphinx/blob/master/sphinx/pygments_styles.py)
+for light mode and the Friendly style for dark mode.
+
+The styles here have been adjusted so that they are WCAG AA compatible. The
+tool at https://github.com/mpchadwick/pygments-high-contrast-stylesheets was
+used to identify colors that should be adjusted.
+
+"""
+from pygments.style import Style
+from pygments.styles.friendly import FriendlyStyle
+from pygments.styles.native import NativeStyle
+from pygments.token import Comment, Generic, Literal, Name, Number, Text
+
+class SphinxHighContrastStyle(Style):
+    """
+    Like Sphinx (which is like friendly, but a bit darker to enhance contrast
+    on the green background) but with higher contrast colors.
+
+    """
+
+    @property
+    def _pre_style(self):
+        # This is used instead of the default 125% so that multiline Unicode
+        # pprint output looks good
+        return 'line-height: 120%;'
+
+    background_color = '#eeffcc'
+    default_style = ''
+
+    styles = FriendlyStyle.styles
+    styles.update({
+        # These are part of the Sphinx modification to "friendly"
+        Generic.Output: '#333',
+        Number: '#208050',
+
+        # These are adjusted from "friendly" (Comment is adjusted from
+        # "sphinx") to have better color contrast against the background.
+        Comment: 'italic #3c7a88',
+        Comment.Hashbang: 'italic #3c7a88',
+        Comment.Multiline: 'italic #3c7a88',
+        Comment.PreprocFile: 'italic #3c7a88',
+        Comment.Single: 'italic #3c7a88',
+        Comment.Special: '#3a7784 bg:#fff0f0',
+        Generic.Error: '#e60000',
+        Generic.Inserted: '#008200',
+        Generic.Prompt: 'bold #b75709',
+        Name.Class: 'bold #0e7ba6',
+        Name.Constant: '#2b79a1',
+        Name.Entity: 'bold #c54629',
+        Name.Namespace: 'bold #0e7ba6',
+        Name.Variable: '#ab40cd',
+        Text.Whitespace: '#707070',
+        Literal.String.Interpol: 'italic #3973b7',
+        Literal.String.Other: '#b75709',
+        Name.Variable.Class: '#ab40cd',
+        Name.Variable.Global: '#ab40cd',
+        Name.Variable.Instance: '#ab40cd',
+        Name.Variable.Magic: '#ab40cd',
+    })
+
+
+
+class NativeHighContrastStyle(NativeStyle):
+    """
+    Like native, but with higher contrast colors.
+    """
+    @property
+    def _pre_style(self):
+        # This is used instead of the default 125% so that multiline Unicode
+        # pprint output looks good
+        return 'line-height: 120%;'
+
+    styles = NativeStyle.styles
+
+    # These are adjusted to have better color contrast against the background
+    styles.update({
+        Comment.Preproc: 'bold #e15a5a',
+        Comment.Special: 'bold #f75050 bg:#520000',
+        Generic.Deleted: '#e75959',
+        Generic.Error: '#e75959',
+        Generic.Traceback: '#e75959',
+        Literal.Number: '#438dc4',
+        Name.Builtin: '#2594a1',
+        # We also remove the underline here from the original style
+        Name.Class: '#548bd3',
+        Name.Function: '#548bd3',
+        # We also remove the underline here from the original style
+        Name.Namespace: '#548bd3',
+        Text.Whitespace: '#878787',
+        Literal.Number.Bin: '#438dc4',
+        Literal.Number.Float: '#438dc4',
+        Literal.Number.Hex: '#438dc4',
+        Literal.Number.Integer: '#438dc4',
+        Literal.Number.Oct: '#438dc4',
+        Name.Builtin.Pseudo: '#2594a1',
+        Name.Function.Magic: '#548bd3',
+        Literal.Number.Integer.Long: '#438dc4',
+    })

doc/src/_static/custom.css

@@ -119,33 +119,6 @@ a:hover {
 /*     text-decoration: underline; */
 /* } */
 
-/* Change the background color of the code blocks (light mode). This should be*/
-/* --color-code-background but setting it in conf.py doesn't work.*/
-.highlight {
-    background-color: hsl(80deg 100% 95%);
-}
-
-/* Improve the color contrast of various syntax highlighted elements in dark*/
-/* mode  */
-
-/* Module names and function and class definitions */
-body:not([data-theme="light"]) .highlight .nn, body:not([data-theme="light"]) .highlight .nf, body:not([data-theme="light"]) .highlight .nc, body:not([data-theme="light"]) .highlight .fm {
-    text-decoration: none;
-    color: #71ADFF;
-}
-/* Numbers */
-body:not([data-theme="light"]) .highlight .mi, body:not([data-theme="light"]) .highlight .mf {
-    color: #51b2fd;
-}
-/* Builtins and self/cls*/
-body:not([data-theme="light"]) .highlight .nb, body:not([data-theme="light"]) .highlight .bp {
-    color: #2fbccd;
-}
-/* Tracebacks */
-body:not([data-theme="light"]) .highlight .gt, body:not([data-theme="light"]) .highlight .gr {
-    color: #ff8888;
-}
-
 /* Disable the fancy scrolling behavior when jumping to headers (this is too
    slow for long pages) */
 html {

doc/src/conf.py

@@ -121,7 +121,9 @@
 #show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+sys.path.append(os.path.abspath("./_pygments"))
+pygments_style = 'styles.SphinxHighContrastStyle'
+pygments_dark_style = 'styles.NativeHighContrastStyle'
 
 # Don't show the source code hyperlinks when using matplotlib plot directive.
 plot_html_show_source_link = False