
Hi @Jack Nairn,
Thank you for posting in this community.
I tested your theme in my side. It seems to be fine.
Perhaps you could try a different browser or computer to see if the site still has the same problem.
I am sorry I do not understand what kind of theme you are hoping to achieve. Perhaps you could use this Theme Generator tool to build your custom theme. You can preview the display of your customized theme in this tool.
Here is the article about configuring and applying a custom theme in SharePoint Online:
How to configure and apply a custom theme in SharePoint Online
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
There are also some sample themes in this post for you to consider:
SharePoint site theming: JSON schema
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