Hi Kasper,
I hope you are well today and thank you for your questions.
reduce the margin/padding of the content pages/posts (white background) and remove the grey background on mobile screens.
Could you please share the screenshots of it describing how it’s displaying on mobile device and how you want it to display so that i can help you to achieve it?
You can use the service http://www.responsinator.com on your desktop browser and then take screenshot of it.
Also share me your site URL.
Aslo, is there any way to add the Site Tagline, but in smaller letters, after the Site-Title?
The solution posted in the following reply will help you to achieve it.
https://colorlibsupport.com/t/tagline-not-showing-in-header/#post-14052
Kind Regards,
Movin