Improve contrast of the specification website

This commit improves the styling of the specification website in order
to meet Level AAA of the Web Content Accessibility Guidelines (WCAG) for
contrast. To this end, this commit modifies the custom CSS file that
overrides some properties of the readthedocs theme for MkDocs:

- Change light blue, orange (monospace) and grey text to the dark blue
of the SPDX logo, and darken the purple text of visited links

- Increase the font size of the footer text

- Change the 'previous' and 'next' links in the section sidebar from
white on light blue to dark blue on white

Signed-off-by: Sebastian Crane <seabass-labrax@gmx.com>

chapters/css/style.css

@@ -21,7 +21,7 @@
   color: #58595b;
 }
 
-.wy-nav-side {
+.wy-nav-side, .rst-versions .rst-current-version {
   background-color: rgb(250, 250, 250);
 }
 
@@ -30,16 +30,20 @@
 }
 
 .wy-breadcrumbs li, .wy-breadcrumbs li a {
-  color: #4597cb !important;
+  color: #00416b !important;
+}
+
+.wy-menu-vertical>ul>li>a {
+  color: #515151;
 }
 
 .wy-menu-vertical li.on a:hover,.wy-menu-vertical li.current>a {
-  background: #4597cb;
+  background: #00416b;
   color: white !important;
 }
 
 .wy-menu-vertical li.on a:hover,.wy-menu-vertical li>a:hover {
-  background: #4597cb !important;
+  background: #00416b !important;
   color: white !important;
 }
 
@@ -51,24 +55,16 @@
   color: #58595b;
 }
 
-.rst-versions {
-  background: #f7f7f7;
+.rst-versions a {
+  color: #00416b !important;
 }
 
-.rst-versions .rst-current-version {
-  background: #4597cb;
-}
-
-div.section a, div.section a:visited {
-  color: #4597cb !important;
-}
-
-h1, h2, h3 {
-  color: #4597cb;
+h1, h2, h3, a {
+  color: #00416b;
 }
 
 code {
-  color: rgb(51, 51, 51);
+  color: #00416b !important;
 }
 
 code, .rst-content tt {
@@ -100,23 +96,15 @@ pre code {
 }
 
 footer {
-  font-size: .6em;
-  color: #d3d3d3;
+  font-size: 1em;
+  color: #515151;
 }
 
 footer a.btn {
   box-shadow: none;
 }
 
-footer div[role=contentinfo] p {
-   font-size: 1.2em;
-}
-
 footer .rst-footer-buttons {
   font-size: 16px;
   color: rgb(64, 64, 64);
 }
-
-footer>a, footer>a:visited {
-  color: #d3d3d3 !important;
-}