Tweak API documentation presentation to match pdoc3's style

This is a generally cleaner style, that avoids using link-like colours
for the API text, has sufficient contrast for all of the content, has a
consistent and distinct font-weight and is visually distinct.

src/furo/assets/styles/content/_api.sass

@@ -3,7 +3,7 @@
 // to select the relevant items.
 
 // API docs!
-dl.py, dl.cpp, dl.c, dl.js
+dl[class]:not(.option-list):not(.field-list):not(.footnote):not(.glossary):not(.simple)
   // Tweak the spacing of all the things!
   dd
     margin-left: 2rem
@@ -40,29 +40,43 @@ dl.py, dl.cpp, dl.c, dl.js
     &:target
       background-color: var(--color-api-highlight-on-target)
 
-dl.class,
-dl.function,
-dl.method,
-dl.enum,
-dl.enum-class,
-dl.enum-struct,
-dl.type,
-dl.exception
-  > dt
-    padding-left: 3em
-    text-indent: -2.5em
+.sig
+  font-weight: bold
+
+  font-size: var(--api-font-size)
+  font-family: var(--font-stack--monospace)
+
+  padding-top: 0.25rem
+  padding-bottom: 0.25rem
+  padding-right: 0.5rem
 
-    // adjust the size of the [source] link on the right.
-    a.reference.internal
-      .viewcode-link
-        width: 3.5rem
+  // These are intentionally em, to properly match the font size.
+  padding-left: 3em
+  text-indent: -2.5em
+
+  border-radius: 0.25rem
+
+  background: var(--color-api-background)
+
+  &:hover
+    background: var(--color-api-background-hover)
+
+  // adjust the size of the [source] link on the right.
+  a.reference.internal
+    .viewcode-link
+      font-weight: normal
+      width: 3.5rem
 
 em.property
   font-style: normal
   &:first-child
     color: var(--color-api-keyword)
+.py.function,
+.py.method
+  > .sig::before
+    content: "def "
+    color: var(--color-api-keyword)
 .sig-name
-  font-weight: bold
   color: var(--color-api-name)
 .sig-prename
   color: var(--color-api-pre-name)
@@ -82,14 +96,3 @@ div.versionadded, div.versionchanged, div.deprecated
 .viewcode-link, .viewcode-back
   float: right
   text-align: right
-
-.sig
-  font-size: var(--api-font-size)
-  font-family: var(--font-stack--monospace)
-  padding: 0.25rem 0.5rem
-  border-radius: 0.25rem
-
-  background: var(--color-api-background)
-
-  &:hover
-    background: var(--color-api-background-hover)

src/furo/assets/styles/variables/_colors.scss

@@ -33,10 +33,10 @@
   --color-api-background: var(--color-background-secondary);
   --color-api-background-hover: var(--color-background-hover);
   --color-api-overall: var(--color-foreground-secondary);
-  --color-api-name: var(--color-brand-content);
-  --color-api-pre-name: var(--color-brand-content);
-  --color-api-paren: var(--color-foreground-primary);
-  --color-api-keyword: var(--color-problematic);
+  --color-api-name: var(--color-problematic);
+  --color-api-pre-name: var(--color-problematic);
+  --color-api-paren: var(--color-foreground-secondary);
+  --color-api-keyword: var(--color-foreground-primary);
   --color-api-highlight-on-target: #ffffcc;
 
   // Inline code background
@@ -125,7 +125,7 @@
 }
 
 @mixin colors-dark {
-  --color-problematic: #e25050;
+  --color-problematic: #ee5151;
 
   // Base Colors
   --color-foreground-primary: #ffffffcc; // for main text and headings
@@ -155,6 +155,7 @@
   --color-guilabel-border: #13395f80;
 
   // API documentation
+  --color-api-keyword: var(--color-foreground-secondary);
   --color-api-highlight-on-target: #333300;
 
   // Admonitions