View the original community article here
Last tested: Apr 3, 2019
Most likely you hitting a scenario where the dashboard already had Embed Settings edited and saved. As per our docs:
If embed settings are applied to a dashboard, they override any custom theme specified in the embed URL. If you want to use a theme for a dashboard that uses embed settings, you can remove the dashboard’s embed settings so that you can use a custom theme.
When is says "remove", this means you have to hit the "Reset to Defaults" option in the Embed Settings page.
After you Reset the settings, you Custom Theme should be applied in full
If not, there may be a typo in your URL!
Color Collection is not being applied?
The visualization color collection needs to be set to Default Visualization Colors in order for a Theme Color Collection to be applied.