Use better contrast colors for links

This removes the separate colors for visited and unvisited. We now use the
darker of the two in light mode and the ligher of the two in dark mode. These
colors satisfy AA color contrast, both against the background and against the
surrounding black/white text (see
sympy#23159).

This replaces the "brand-content" color in light mode since this is primarily
used for links. We still use the lighter color for the API function names in
light mode, so that they pop out more (dark mode already has a lighter color
for API function names).

doc/src/_static/custom.css

@@ -98,22 +98,22 @@ dl.py .field-list dt, dl[class]:not(.option-list):not(.field-list):not(.footnote
     font-size: 120%;
 }
 
-/* Disable underlines on links except on hover, and give visited links a*/
-/* darker green color. */
+/* Disable underlines on links except on hover */
 a {
     text-decoration: none;
 }
-a:visited {
-    color: #307748;
-    text-decoration: none;
-}
 a:hover {
     text-decoration: underline;
 }
-a:visited:hover {
-    color: #307748;
-    text-decoration: underline;
-}
+/* TODO: Some styling for visited? */
+/* a:visited { */
+/*     color: #307748; */
+/*     text-decoration: none; */
+/* } */
+/* a:visited:hover { */
+/*     color: #307748; */
+/*     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.*/

doc/src/conf.py

@@ -160,7 +160,7 @@
 common_theme_variables = {
     # Main "SymPy green" colors. Many things uses these colors.
     "color-brand-primary": "#81B953",
-    "color-brand-content": "#29A329",
+    "color-brand-content": "#307748",
 
     # The left sidebar.
     "color-sidebar-background": "#3B5526",
@@ -170,13 +170,12 @@
     "color-sidebar-item-background--hover": "#8AB950",
     "color-sidebar-item-expander-background--hover": "#8AB950",
 
+    "color-link-underline--hover": "var(--color-link)",
     "color-api-keyword": "#000000bd",
-    "color-api-name": "var(--color-brand-content)",
+    "color-api-name": "#29A329",
     "color-api-pre-name": "var(--color-brand-primary)",
     "api-font-size": "var(--font-size--normal)",
     "color-foreground-secondary": "#53555B",
-    "color-link-hover": "var(--color-link)",
-    "color-link-underline--hover": "#29A329",
 
     # TODO: Add the other types of admonitions here if anyone uses them.
     "color-admonition-title-background--seealso": "#CCCCCC",
@@ -201,6 +200,8 @@
     # The dark variables automatically inherit values from the light variables
     "dark_css_variables": {
         **common_theme_variables,
+        "color-brand-content": "#29A329",
+        "color-link": "var(--color-brand-content)",
         "color-api-keyword": "#FFFFFFbd",
         "color-api-overall": "#FFFFFF90",
         "color-api-paren": "#FFFFFF90",