Change the tab text colour to black.

Keeping the main title white and bold for now as it pops more. Maybe also
need to change.

Fixes #25.

src/Journal/index.tsx

@@ -83,7 +83,7 @@ class Journal extends React.PureComponent<PropsTypeInner> {
         <SecondaryHeader text={collectionInfo.displayName} />
         <Tabs
           fullWidth={true}
-          style={{ backgroundColor: theme.palette.primary.main, color: 'white' }}
+          style={{ backgroundColor: theme.palette.primary.main }}
           value={currentTab}
           onChange={(event, tab) => this.setState({ tab })}
         >

src/Pim/PimMain.tsx

@@ -96,7 +96,7 @@ class PimMain extends React.PureComponent<PropsType> {
       <React.Fragment>
         <Tabs
           fullWidth={true}
-          style={{ backgroundColor: theme.palette.primary.main, color: 'white' }}
+          style={{ backgroundColor: theme.palette.primary.main }}
           value={tab}
           onChange={(event, value) => this.setState({tab: value})}
         >