Enable alternative themes for Bootstrap (e.g. Boostwatch)

Alchemy is a great theme, but it was not using the full potential of
underlying technology. Bootstrap provides an easy way to customize look
and feel of rendered pages, and a lot of themes for Bootstrap are freely
available, e.g. at <https://boostwatch.com>

This commit introduces a new configuration variable BOOTSTRAP_CSS to make
use of any existing Bootstrap skins. All incompatibilities in `theme.css`
were fixed:

- Instead of using constant color values we use Bootstrap css variables for
  colors:
  <https://getbootstrap.com/docs/4.3/getting-started/theming/#css-variables>
- The only hardcoded color value left is the color for header/footer
  borders. It was moved into a variable `--alchemy-border` for easier
  overriding if needed. I have tested the current value against both dark
  and light Boostwatch themes, it works ok.
- Pagination template was slightly changed to avoid producing invisible
  text with some of Bootstrap skins

alchemy/static/css/theme.css

@@ -1,24 +1,14 @@
+:root {
+    --alchemy-border: rgba(0,0,0,.1);
+}
 ::selection {
-  background: #0085a1;
-  color: #fff;
+  background: var(--primary);
+  color: var(--secondary);
   text-shadow: none;
 }
 
-body {
-  background-color: #f5f5f5;
-}
-
-a {
-  color: #333;
-}
-
-a:focus,
-a:hover {
-  color: #0085a1;
-}
-
 blockquote {
-  color: #818a91;
+  opacity: .7;
   font-style: italic;
 }
 
@@ -38,7 +28,7 @@ hr {
 }
 
 .header {
-  border-bottom: 1px solid rgba(0,0,0,.1);
+  border-bottom: 1px solid var(--alchemy-border);
 }
 
 .header img {
@@ -62,23 +52,18 @@ hr {
 }
 
 .main {
-  background-color: #fff;
   padding: 1.5rem 0;
 }
 
 .footer {
-  border-top: 1px solid rgba(0,0,0,.1);
+  border-top: 1px solid var(--alchemy-border);
 }
 
 .highlight pre {
-  border: 1px solid rgba(0,0,0,.1);
+  border: 1px solid var(--alchemy-border);
   padding: 1rem;
 }
 
-.pagination .page-link {
-  color: #333;
-}
-
 .teaser header ul {
   list-style: none;
   padding-left: 0;