A design with white as the base color, matching the banner.
Literal
Banner [with-と] same, white [object-を] base-tone [as-とした] design.
バナー 'banner' — a web design loanword. 基調とした 'with X as the base tone' — 基調 'keynote, base tone' + とする. 白を基調としたデザイン is standard design/web vocabulary: 'a white-based design.' A sentence fragment describing a visual spec.