Hello,
No it will cause conflict and it is wrong to copy paste everything from another template to custom.css
also, dont modify any CSS/HTML templates as this will always cause the theme not to function properly on new IPS versions.
Use custom.css for ALL your styling and customization's you can literally do everything you want with styles and without touching a single other css template 😉
How to use custom.css? Say for example you edited a css template to change a color. you dont need to do that! just copy the css for the color and paste it inside custom.css then change the color code
There is no no need to enable designer mode editing javascript to change a font.
simply use css to change font
you can also just post how to change a font here in forums, im sure there is always someone to help. you could also ask it here in my support topics i can always help with small customizations like font/color text size or whatever 🙂
Thank you too!
be safe