I see it in light theme as well on ubuntu 18.0.4
VirtualBox_2018-05-31_09-56-39.png (image/png), 633.20 KB, created by Wim Jongman CLA on 2018-05-31 03:57:01 EDT
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Creator: Wim Jongman CLA
Created: 2018-05-31 03:57:01 EDT
Size: 633.20 KB
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Attachments on bug 535123: 274197 | 274238 | 274272