Fix color scheme of buttons in /upgrade

Originally created by @Anonymous on #15597 (Redmine)

- on /upgrade buttons look weird: they have two background colors, green and blue.
- on the homepage the title border looks ugly

I’m proposing a branch to fix these two issues, as people started wondering. Please review and cherry-pick what you like.

Feature Branch: web/15597-upgrade-buttons

Attachments

• install.png
• upgrade.png

Related issues

• Related to #11493 (closed)
• Related to #15114 (closed)