Encountering the ‘Invalid Theme’ button
Not too long ago, Tumblr introduced a system that checks a theme’s HTML, if the system deems the HTML invalid, they make it seem near impossible to work with the theme.
But the criteria for invalid HTML is not clear, what deems HTML to be ‘invalid’? It’s safe to assume that this is a hit and miss feature, especially when it comes to custom built themes (themes not listed in Tumblr’s theme garden).
So what does it mean? Well it essentially means nothing. When you install a custom theme, in this case Kinley or Polarmark, you may encounter it the first time you paste the HTML into the editor pane. The button’s function doesn’t change however, clicking it will still update the theme’s preview. You can continue on and click save, reload the page and you’ll unlikely see the button again.
It’s safe to ignore this button, it does not actually affect the ability to update and save the theme. We can promise you that my themes’ HTML is valid and working and we’re dedicated to keeping the custom themes we release up-to-date.