Change:
.monaco-workbench .part.editor > .content .editor-group-container.active > .title .tabs-container > .tab.sticky.dirty > .tab-actions .action-label:not(:hover)::before, .monaco-workbench .part.editor > .content .editor-group-container > .title .tabs-container > .tab.sticky.dirty > .tab-actions .action-label:not(:hover)::before { content: "ebb2"; /* use `pinned-dirty` icon unicode for sticky-dirty indication */ <ins> font-family: 'codicon'; </ins> } .monaco-workbench .part.editor > .content .editor-group-container.active > .title .tabs-container > .tab.dirty > .tab-actions .action-label:not(:hover)::before, .monaco-workbench .part.editor > .content .editor-group-container > .title .tabs-container > .tab.dirty > .tab-actions .action-label:not(:hover)::before { content: "ea71"; /* use `circle-filled` icon unicode for dirty indication */ <ins> font-family: 'codicon'; </ins> } .monaco-workbench .part.editor > .content .editor-group-container > .title .tabs-container > .tab.active > .tab-actions .action-label,

Discussion:
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We need to avoid hardcode characters in CSS. So, if possible remove. If not easily possible, add ` rule. It looks like there's a close icon set that is properly themed and that sets the font family.
There is a bit of a trick going on here. We are overwriting the close codicon with the dirty codicon when the file is dirty and the action is not being hovered.

It's rather complex to solve this without hardcoding the value in css. I'll temporarly fix it by adding the font family and I'll try to think of a more elegant solution for this.