Style suggestions for sphinx13

As a suggestion:

- increase paragraph line-height to 1.5 to give text a bit more room to breathe.
  1.5 is a common choice in web page typography; e.g. rtd-sphinx-theme, furo and
  GitHub use this (pydata sphinx theme goes even to 1.65).
- Don't shrink the text in admonitions. They are equally important and thus should
  have the same size as regular paragraphs.
- Adapt the border color for all warning/error like admonitions. Warnings had a
  fixed color that did not fit well to the new admonition style.

doc/_themes/sphinx13/static/sphinx13.css

@@ -241,6 +241,7 @@ div.footer a {
 
 p {
     margin: 0.8em 0 0.5em 0;
+    line-height: 1.5;
 }
 
 a {
@@ -395,7 +396,6 @@ table td, table th {
 }
 
 div.admonition, div.warning {
-    font-size: 0.9em;
     margin: 1em 0 1em 0;
     border: 1px solid #86989B;
     border-radius: var(--admonition-radius);
@@ -451,6 +451,7 @@ p.admonition-title::before {
     content: "";
     height: 1rem;
     left: .5rem;
+    top: .5rem;
     position: absolute;
     width: 1rem;
     background-color: #5f5f5f;
@@ -516,9 +517,15 @@ div.warning > p.admonition-title::before {
     -webkit-mask-image: var(--icon-warning);
     mask-image: var(--icon-warning);
 }
-
+div.caution,
+div.important,
 div.warning {
-    border: 1px solid #940000;
+    border-color: var(--colour-warning-fg);
+}
+div.attention,
+div.danger,
+div.error {
+    border-color: var(--colour-error-fg);
 }
 
 div.admonition > ul,