Theme addons are now supported in Construct 3 r38+. These are essentially custom stylesheets you apply to the document Construct 3 is running in. This allows complete control over the appearance of the Construct 3 editor.
The SDK documentation has been updated to cover theme addons:
Most of the new content is in the themes section.
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Never mind themes, please finish the plugin SDK.
Updated the theme download to v1.0.1, which adds some new CSS variables: --default-layer-color, --c3logo-type, --layoutview-viewport-dash-size, --layoutview-collisionpoly-color, --layoutview-collisionpoly-width