Very weird behaviour after transfer of theme

Hi all

I created a theme for a client on our own dev HubSpot account and everything was working as intended.
After i transfered the theme to the clients account - which by the way had no prior website theme(s) installed - I’m experiencing that a lot of the styling is being overwritten by “something” which I can’t seem to figure out what is.

Like, the H1 styling is being overwritten, the default padding for .dnd-section is being overwritten etc. I have to force the styling through with !important before it works. Obviously this is not the way to fix it.
The H1 font is defined as “Poppins” in the stylesheet, but is overwritten by “Nunito Sans” !? The default header in the Theme settings is Merryweather, which we don’t use, that only makes this even more strange.
But since there are not other themes installed, why are our styles being overwritten, and what’s the source of the behaviour?
When the editor first loads, everything looks ok, then after 2 seconds the styling changes (something seems to load delayed).
I truly hope someone knows what’s going on :confused:
I primarly have access to the CMS part of the clients HubSpot account, plus Design Tool and CMS settings (domains, theme etc.). Is something hiding in plain sight?
Lastly i’m not allowed to publish pages on one of the temporary domains so I can see how the pages look before going live.

Should add that there’s a @hubspot folder with a Growth theme in it. Is that the one causing trouble and if so, how and why? It’s not being used in the CMS section (no CMS pages existed before i created one with the transfered theme).
These are the default @hubspot themes in our dev account and they don’t cause any issues:

I should note that resetting the theme settings through the editor does not change anything (and we don’t really use those settings actively anyway).
Also the theme we’ve transfered does have a theme-overrides.css file in which we can add changes that will update in the editor window. However the style overwrites we experience isn’t coming from that. file

Oh well, to anyone who might experience this, make sure to clear console errors first.
In my case and old folder reference wasn’t updated and caused all of this. I guess it falls back on the default hubspot theme if it can’t render everything properly?