Fix dark theme to not have dark text color that is same as background (…

…#456)

Fixes: rapidsai/cudf#14463

This PR fixes dark theme text color to not pick the background color and
also fixes span tags text color to not pick general body color.

Previously:



Now:
<img width="886" alt="Screenshot 2023-11-21 at 7 56 59 PM"
src="https://github.com/rapidsai/docs/assets/11664259/f243e7d1-90fe-47ab-b897-e313c9a2ca10">

assets/css/custom.css

@@ -182,7 +182,7 @@ body {
     roboto, noto, "segoe ui", arial, sans-serif;
   font-size: inherit;
   line-height: 1.4;
-  color: #5c5962;
+  color: var(--pst-color-text-base);
 }
 
 h1,
@@ -546,7 +546,7 @@ table.io-supported-types-table thead{
 }
 
 :root {
-
+  --pst-heading-color: var(--pst-color-primary);
   --pst-color-active-navigation: 114, 83, 237;
   --pst-color-navbar-link: 77, 77, 77;
   --pst-color-navbar-link-hover: var(--pst-color-active-navigation);