The "Classic" in the Classic theme refers to the default look of Redmine 0.5.1 and earlier. According to the changelog, the theme was added in Redmine 0.6.1, after the default theme was changed in Redmine 0.6.0, to provide a look similar to the default theme in Redmine 0.5.1 and earlier.
This issue proposes replacing the top menu icons, which are an important part of the Classic theme's look, with SVG icons that look very different from the current raster icons. I think this would significantly change the nature of the Classic theme, which is intended to reproduce the look of Redmine 0.5.1 and earlier.
Personally, I would prefer to keep the current raster icons in the Classic theme's top menu to keep its look close to Redmine 0.5.1 and earlier.
If the icons are changed to SVG icons, I think the change should target Redmine 7.1.0 or later and should not be backported to a minor release such as Redmine 6.1.4, because it would significantly change the look of the Classic theme.
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