We recently updated our OrganicWeb Mailchimp services website and Mailchimp training websites. Mailchimp make available their brand assets however we chose to go along the route of Google material design guidelines (mostly).
I’m not a designer however I just love the look of material design. Added to that is the familiarity most people on the internet have with the design of Google and their many websites and Android devices.
Because material design is so popular there are also many easy to use resources for designing. For example a tool that I seem to use alot lately is Material Palette which makes choosing colors simple.