[website] Revise some colors in darkmode

Summary:
- Set constant colors for the "hero" and "whoUses" sections.
- Use brighter purple color for links.

Reviewed By: dulmarod

Differential Revision: D58873780

fbshipit-source-id: f9544cffe58164f5e9380bc95b83be76ad5700b2

website/src/css/custom.css

@@ -6,7 +6,8 @@
  */
 
 :root {
-  --ifm-color-primary: #6223b0;
+  --infer-purple: #6223b0;
+  --ifm-color-primary: var(--infer-purple);
   --ifm-color-primary-dark: #58209e;
   --ifm-color-primary-darker: #4e1c8d;
   --ifm-color-primary-darkest: #45197b;
@@ -17,10 +18,19 @@
   --ifm-code-font-size: 95%;
 }
 
+[data-theme='dark'] {
+  --ifm-color-primary: var(--ifm-color-primary-lightest);
+}
+
 .large-math {
   vertical-align: text-bottom;
 }
 
+.hero {
+  --ifm-hero-text-color: var(--ifm-color-gray-0);
+  --ifm-hero-background-color: var(--infer-purple);
+}
+
 /* Announcement banner */
 
 :root {

website/src/pages/styles.module.css

@@ -56,14 +56,14 @@
 }
 
 .whoUses {
-  color: var(--ifm-font-color-base-inverse);
-  background-color: var(--ifm-color-primary);
+  color: var(--ifm-color-gray-0);
+  background-color: var(--infer-purple);
   padding: 4rem 0;
   text-align: left;
 }
 
 .whoUses a {
-  color: var(--ifm-font-color-base-inverse);
+  color: var(--ifm-color-gray-0);
 }
 
 .whoUsesItems {