Forum themes CSS/Bootstrap bug

[Mototroller]

There some issues with forum styles on my Linux machine (Windows seems to be ok). Default (toxic light theme) works fine, any other causes layout problems (see screenshot).

Browsers details:

4.4.0-112-generic #135-Ubuntu x86-64
Google Chrome Version 64.0.3282.140 (Official Build) (64-bit)
Firefox 58.0.1 (64-bit)

Screenshot:

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Questionable style lines inside bootstrap.min.css:

.navbar-nav {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -ms-flex-direction: column;
  flex-direction: column; /* <= this one */
  padding-left: 0;
  margin-bottom: 0;
  list-style: none;
}

body {
    margin: 0;
    font-family: "Roboto", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    font-size: 0.875rem; /* <= font becomes unexpectedly small */
    ...
}

I have no idea how these styles been generated, but seems like there are some problems here.

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UPD: cache cleaning doesn't help, as well as even browser changing.

[MrFaul]

I have it as well, only the default/paper works...
And I have a Windows machine / chrome