Fix bad contrast in the platform pill in the docsite (#1251)

White text on green breaks site accessibility standards so I'm updating
the active text color for the platform picker pill to be
`--ifm-color-secondary-contrast-foreground`, which is white on light
mode and light grey on dark mode.

docs/src/components/platformcontext.css

@@ -22,7 +22,7 @@
 
 .pill-option.active {
     background-color: var(--ifm-color-primary);
-    color: var(--ifm-color-secondary-lightest);
+    color: var(--ifm-color-secondary-contrast-background);
     font-weight: bold;
 }