Custom SCSS polish pass

[gjong]

Labels: design, priority:P3
Parent issue: Epic: Documentation site overhaul

Problem
Several small visual issues: body font weight is quite light (Inter 300), TIP/WARNING/NOTE callouts aren't styled distinctly, screenshots have no frame treatment, and the content column could be wider on large screens. Dark mode hasn't been verified for logo/screenshot contrast.

Suggested fix

• Add assets/scss/custom.scss covering: body font weight (400), styled callouts, screenshot frames (border-radius/shadow), wider content column on large screens.
• Test dark mode for logo and screenshot visibility.

Acceptance criteria

• Custom SCSS added and applied site-wide.
• Dark mode manually verified.