メインのCSSコードを変更するにはどうすればいいですか？

[merefield (Robert)]

ここから始めるのが一番です。

Developing Discourse Themes & Theme Components

Discourse Themes and Theme Components can be used to customize the look, feel and functionality of Discourse’s frontend. This section of the developer guides aims to provide all the reference materials you need to develop simple themes for a single site, right up to complex open-source theme components. This introduction aims to provide a map of all the tools and APIs for theme development. If you prefer a step-by-step tutorial for theme development, jump straight to: Themes vs. Theme Compon…

サイトの速度が遅くなることはありません。