Move theme to this file.

src/App.style.ts

+import { createMuiTheme } from '@material-ui/core/styles';
+
+export const muiTheme = createMuiTheme({
+  palette: {
+    primary: {
+      main: '#5e5ce4',
+    },
+    secondary: {
+      main: '#e2e45c',
+    },
+  },
+
+  typography: {
+    fontSize: 16,
+  },
+
+  overrides: {
+    MuiTabs: {
+      indicator: {
+        backgroundColor: 'white',
+      },
+    },
+    MuiTab: {
+      wrapper: {
+        flexDirection: 'row',
+      },
+    },
+  },
+});