Add .theme file to the PurgeCSS config

Ensures that any classes in `opdavies.theme` are not purged in a
production CSS build.

References #341

web/themes/custom/opdavies/tailwind.config.js

@@ -12,6 +12,7 @@ module.exports = {
     content: [
       '../../../../config/**/*.yml',
       'body-field-values.txt',
+      'opdavies.theme',
       'templates/**/*.twig'
     ],
     options: {