There are a lot of things to consider when you want an element's layout to look the same even when using different themes:
1.) Specificity
- Research what specificity is all about and how you can use it to your advantage
- If you want to make your StyleSheet more dominant, place the tag after the less dominant sheets.
- Make sure your class names are not used by others - trick: use class prefixes
- If all else fails, the !important keyword is your friend.
2.) Parent Layouts
- Say the playbutton's parent element is affected by the theme and that the #playbutton is using percentages, chances are the button will take on the parent's size as it is still dependent. It would be easier if the parent is not affected by the theme so you may need to go back to #1.
I don't know what the total markup of this project's page but I hope this helps.