Just chiming in with some feedback because there’s been a few posts recently looking for an expert with a budget of a few hundred dollars…
$100-150 is a medior hourly rate for a developer. It will be below for junior developers and above for senior experts.
The theme you linked to as an example is not doing anything super fancy, but it’s carefully crafted. Also, the host company has a fully defined design language in place, so the theme developer didn’t have to come up with style definitions themselves.
I’d probably estimate around 10-15 hrs to build such a theme giving an example template. If you don’t have a design language defined this adds at least another couple hours for very basic definitions of colors, fonts, borders.. When done for a client, you also have to add in time for project management, communication and deployment of the theme, let’s say just 5 hrs. So as a ballpark I’d be looking at a project range of 20-25 hrs. Multiply this by $150 and you get a rough scope where I for example would start budgeting such a project.