Alert boxes are displayed with an inconsistent style

In 0.99.129.1.1-rc, alert boxes are displayed with an inconsistent style, whereas they are displayed just fine in the current beta:

How they show in 0.99.128.2.1-beta:

0.99.129.1.1-rc

Sequoia (15)

Pop-ups are already looking better in 0.99.129.3-rc, but that persistent scrollbar is looking a bit funny:

This seems to be fixed in 0.99.129.4-rc 🎉