Add beets.css customizing Sphinx theme "pydata"

- Set static path and css file in Sphinx config.
- Codebox style to black/white
- Secondary color to beetroot red
- Inline code color to beetroot green
- Leave primary color (pydata teal), difficult to read with beetroot
  green. Works well with beetroot red actually.
- Leave some notes on colors in beets.css for reference.

docs/_static/beets.css

@@ -0,0 +1,12 @@
+html[data-theme="light"] {
+    --pst-color-secondary: #a23632;
+}
+html[data-theme="light"] {
+    --pst-color-inline-code: #a23632;
+}
+
+/* beetroot red: #a23632 */
+/* beetroot green: #1B5801 */
+/* beetroot green light: rgb(27, 150, 50) */
+/* pydata teal (primary): #126A7E */
+/* pydata violet (secondary): #7D0E70 */

docs/conf.py

@@ -66,6 +66,9 @@ html_theme_options = {
     "logo": {
         "text": "beets",
     },
+    "pygment_light_style": "bw",
 }
 html_title = "beets"
 html_logo = "_static/beets_logo.png"
+html_static_path = ['_static']
+html_css_files = ['beets.css']