Theme development

I'm working on implementing my site in Drupal 8 and have decided to use the Danland theme. That theme comes with some images that I would like to replace, there will probably be other changes (css?) that I will want to make. I've already had composer replace all of my changes with the original content so updating it in place probably is not the way to go. What's the best practice to proceed? I attempted to use danland as a base theme but ran into errors early on that I'm not sure how to address, they have to do with the template_preprocess_* hooks.

The organization I work for is looking for some guidance on choosing a base theme to develop a new responsive theme from. The requirements are the following:

Im trying to replace some php code in .php.plp file to change it to a html.twig file, i dont know if im able to replace all the logic with just twig when i have php code deprecated or drupal functions wich one i cant use inside twig tags, and of course i can't use php and twig on same file (at least with my knowledge).The php.plp code is 

Hi,

I would like to render one of my forms built with the webform module in my twig templates, how can I do this?

Thank you

Hi, I have a webform with a Document File field. In my twig email template, I want to test if that field is empty. For text area fields for example, this syntax works :