- simplify CSS in index.css
- fix start-page layout
- reduce timeline-track height
- fix timeline-track legend layout
- fix scrollbar colors in dark theme

Bug: v8:10644
Change-Id: If3bb7422e6866bac766e7851f489a42ecbcf1d78
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2463239
Auto-Submit: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70438}