refactor: Replace white & black colors with theme aware ones

Signed-off-by: Carlos Esteban Lopez <lcarlosesteb@vmware.com>

plugins/techdocs/src/reader/components/TechDocsReaderPageHeader/TechDocsReaderPageHeader.tsx

@@ -19,6 +19,7 @@ import Helmet from 'react-helmet';
 
 import { Grid } from '@material-ui/core';
 import { Skeleton } from '@material-ui/lab';
+import { useTheme } from '@material-ui/core/styles';
 import CodeIcon from '@material-ui/icons/Code';
 
 import {
@@ -64,6 +65,11 @@ export type TechDocsReaderPageHeaderProps = PropsWithChildren<{
 export const TechDocsReaderPageHeader = (
   props: TechDocsReaderPageHeaderProps,
 ) => {
+  const {
+    palette: {
+      common: { white },
+    },
+  } = useTheme();
   const { children } = props;
   const addons = useTechDocsAddons();
   const configApi = useApi(configApiRef);
@@ -138,7 +144,7 @@ export const TechDocsReaderPageHeader = (
               container
               direction="column"
               alignItems="center"
-              style={{ color: '#fff' }}
+              style={{ color: white }}
             >
               <Grid style={{ padding: 0 }} item>
                 <CodeIcon style={{ marginTop: '-25px' }} />