DOC: Improve internal sphinx13 style (#12439)

This commit adds a few changes to the theme to make it more readable and visually more appealing. In particular:

- Slightly increase the font weight of headings (300 --> 400).
- Adapt the color of the search box border to sphinx-blue.
- Add horizontal bars between the components of the side bar. 
- Re-style admonitions to use colored title backgrounds.

doc/_themes/sphinx13/static/sphinx13.css

@@ -4,6 +4,10 @@
 :root {
     --fonts-sans-serif: system-ui, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
     --colour-sphinx-blue: #0A507A;
+    --colour-warning-bg: #f8e3d0;
+    --colour-note-bg: #dce7fc;
+    --colour-success-bg: #d6ece1;
+    --colour-todo-bg: #e0c7ff;
     --colour-text: #333;
     --colour-links-light: #057;
 }
@@ -93,6 +97,16 @@ div.sphinxsidebar {
     font-size: 1em;
 }
 
+/* horizontal line between sidebar components */
+div.sphinxsidebar div:not(:first-child) {
+    border-top: 1px solid var(--colour-sphinx-blue);
+}
+
+/* overwrite color from basic theme */
+div.sphinxsidebar input {
+    border: 1px solid var(--colour-sphinx-blue);
+}
+
 div.sphinxsidebar h3 {
     font-size: 1.5em;
     margin-top: 0;
@@ -186,7 +200,7 @@ h1 {
     margin: 10px 0 0 0;
     font-size: 2.4em;
     color: var(--colour-sphinx-blue);
-    font-weight: 300;
+    font-weight: 400;
 }
 
 h1 span.pre {
@@ -198,15 +212,15 @@ h1 span.pre {
 h2 {
     margin: 1em 0 0.2em 0;
     font-size: 1.5em;
-    font-weight: 300;
+    font-weight: 400;
     padding: 0;
     color: #174967;
 }
 
 h3 {
     margin: 1em 0 -0.3em 0;
     font-size: 1.3em;
-    font-weight: 300;
+    font-weight: 400;
 }
 
 div.body h1 a, div.body h2 a, div.body h3 a, div.body h4 a, div.body h5 a, div.body h6 a {
@@ -341,8 +355,32 @@ div.admonition > pre, div.warning > pre {
 div.admonition > p.admonition-title,
 div.warning > p.admonition-title {
     font-weight: bold;
+    background-color: #ddd;
+    margin: -1rem -1rem 0.8rem -1rem;
+    padding: 0.3rem 1rem;
 }
 
+div.important > p.admonition-title,
+div.caution > p.admonition-title,
+div.warning > p.admonition-title {
+    background-color: var(--colour-warning-bg);
+}
+
+div.note > p.admonition-title {
+    background-color: var(--colour-note-bg);
+}
+
+div.hint > p.admonition-title,
+div.tip > p.admonition-title,
+div.seealso > p.admonition-title {
+    background-color: var(--colour-success-bg);
+}
+
+div.todo > p.admonition-title {
+    background-color: var(--colour-todo-bg);
+}
+
+
 div.warning {
     border: 1px solid #940000;
 }