[metrics-bugs] #25404 [Metrics/Website]: Update design to use new styleguide (bootstrap 4)

[Tor Bug Tracker & Wiki]

#25404: Update design to use new styleguide (bootstrap 4)
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     Reporter:  irl              |      Owner:  metrics-team
         Type:  project          |     Status:  new
     Priority:  Medium           |  Milestone:
    Component:  Metrics/Website  |    Version:
     Severity:  Normal           |   Keywords:  ux-team
Actual Points:                   |  Parent ID:
       Points:                   |   Reviewer:
      Sponsor:                   |
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 https://styleguide.torproject.org/ provides a Bootstrap fork customised
 for Tor Project to provide a consistent visual identity across
 subprojects. Bootstrap 3 is no longer maintained and at some point we will
 need to update to Bootstrap 4 anyway to avoid any brokenness in browsers
 that may appear (we have already seen brokenness that we've had to patch
 manually in #25254).

 While implementing this, we may also choose to make other design changes
 or to reorganise navigation on the site. It would probably be worthwhile
 to hold off making any large changes to the current structure of the site
 until it has been considered how it would work with the new styles.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25404>
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