refactor: extract themes to their own folder

ui/src/themes/dark.js

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+import blue from '@material-ui/core/colors/blue'
+
+export default {
+  palette: {
+    primary: {
+      main: '#90caf9'
+    },
+    secondary: blue,
+    type: 'dark'
+  },
+  overrides: {
+    MuiFormGroup: {
+      root: {
+        color: 'white'
+      }
+    }
+  }
+}

ui/src/themes/index.js

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+import LightTheme from './light'
+import DarkTheme from './dark'
+
+export { LightTheme, DarkTheme }

ui/src/themes/light.js

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+export default {
+  palette: {
+    secondary: {
+      light: '#5f5fc4',
+      main: '#283593',
+      dark: '#001064',
+      contrastText: '#fff'
+    }
+  },
+  overrides: {
+    MuiFilledInput: {
+      root: {
+        backgroundColor: 'rgba(0, 0, 0, 0.04)',
+        '&$disabled': {
+          backgroundColor: 'rgba(0, 0, 0, 0.04)'
+        }
+      }
+    }
+  }
+}