preferences.css

#better-bibtex-export-list listheader { padding-left: 0px; }
#better-bibtex-export-list listitem { padding-left: 20px; }
#better-bibtex-export-list listitem.export-state-error    { background:url("auto-export/error.svg")   center left no-repeat; background-size: 18px 18px;}
#better-bibtex-export-list listitem.export-state-pending  { background:url("auto-export/pending.svg") center left no-repeat; background-size: 18px 18px;}
#better-bibtex-export-list listitem.export-state-running  { background:url("auto-export/running.svg") center left no-repeat; background-size: 18px 18px;}
#better-bibtex-export-list listitem.export-state-done     { background:url("auto-export/done.svg")    center left no-repeat; background-size: 18px 18px;}

.scrollable {
  overflow: auto;
}
.hidden {
  display: none;
}

label {
  border: 2px solid red;
}

preferences.xhtml

<vbox>
  <html:link href="chrome://zotero-better-bibtex/skin/preferences.css" type="text/css" rel="stylesheet"></html:link>
  <popupset>
    <tooltip id="tooltip-bibtexURL">
      <description>&better-bibtex.Preferences.export.fields.doi-and-url.warning;</description>
    </tooltip>
    <tooltip id="tooltip-jabrefFormat">
      <description>&better-bibtex.Preferences.export.jabrefFormat.warn;</description>
    </tooltip>
    <tooltip id="tooltip-importSentenceCase">
      <description>&better-bibtex.Preferences.advanced.import.sentenceCase.warning;</description>
    </tooltip>
    <tooltip id="tooltip-importCaseProtection">
      <description>&better-bibtex.Preferences.advanced.import.caseProtection.warning;</description>
    </tooltip>
    <tooltip id="tooltip-exportTitleCase">
      <description>&better-bibtex.Preferences.advanced.export.titleCase.warning;</description>
    </tooltip>
    <tooltip id="tooltip-exportBraceProtection">
      <description>&better-bibtex.Preferences.advanced.export.braceProtection.warning;</description>
    </tooltip>
    <tooltip id="tooltip-retainCache">
      <description>&better-bibtex.Preferences.advanced.export.retainCache.warning;</description>
    </tooltip>
  </popupset>
  <deck id="better-bibtex-prefs-deck" selectedIndex="0">
    <label id="better-bibtex-loading">&better-bibtex.Preferences.prefpane.loading;</label>
  </deck>
</vbox>