Fix: Allow code base color to be customized with theme config (#1335)

- Add `theme.color.codeBase` property
- Allow syntax highlight default color to set without affecting base color
- The codeBase and codeBackground can be set to allow finer control on code text contrast ratios. e.g. Use dark themes for code samples.

src/client/styles/prismTheme.js

@@ -1,4 +1,7 @@
 const prismTheme = ({ color }) => ({
+	'&': {
+		color: color.codeBase,
+	},
 	[`& .token.comment,
 & .token.prolog,
 & .token.doctype,

src/client/styles/theme.js

@@ -26,6 +26,7 @@ export const color = {
 	ribbonBackground: '#e90',
 	ribbonText: '#fff',
 	// Based on default Prism theme
+	codeBase: '#333',
 	codeComment: '#6d6d6d',
 	codePunctuation: '#999',
 	codeProperty: '#905',