must third party themes follow PrestaShop specific guidelines?

[ballashop]

Hi,
I purchased a theme in the marketplace but I noticed that the theme does not show the third-party and native PrestaShop modules well.
Question:
Should third-party themes follow specific PrestaShop guidelines?
Let me explain better with an example:
We purchased the theme that shows native themes very badly, a purchased cross selling module is not displayed as it should.
The developer of the theme asks us to integrate these changes by paying. is it correct?
Or is it a malfunction of the theme that the developer needs to improve compatibility for?
Or none of these?
Thanks for any reply.
Balla

[ballashop]

Hi,

The developer of a purchased module gave us an answer:
The modules are developed according to the characteristics of the native theme, so some third-party themes may not be 100% compatible with those modules.

I'll explain better:
some CSS rules, which are specifically created for the theme in use, for some modules could create layout problems. 

PrestaShop is a widely used platform with many modules and themes, I understand that sometimes compatibility problems may arise both at the code and at the graphic level.
In fact I have found many problems for both.
I'm here to ask the PrestaShop community for useful tips to better understand how to move around PrestaShop modules and themes.

Thanks for any reply.

Balla