Custom icons should be permitted

[rdanter]

Problem

Previously it was possible to install custom icon sets and themes. This has recently been changed so that in some places, such as the Applications menu, it is no longer possible to use these icons.

Other areas, such as the Dock, still allow custom icons, which makes the Applications menu inconsistent.

Proposal

Please restore the ability to have cusom icons.

Prior Art (Optional)

No response

[ryonakano]

This is due to 476a9fd

[danirabbit]

It looks in daily that changing the icons from elementary to Adwaita breaks:

• Sound indicator volume level icons
• gresource icons the stylesheet depends on granite#666
• Bluetooth status icons use old namespace panel-bluetooth#211
• Placeholder icon missing when using other icon sets panel-power#254
• text size icons for quick settings
• status emblems in Bluetooth (will be automatically fixed in GTK4)

I'm filing some issues against the specific indicators where they are responsible, and we need to finish gresourcing indicator-specific icons in those indicators like we've started doing with notifications, bluetooth, etc. And then when we've solved the breakage we can remove the hardcoding so that third-party and fd.o namespace icons can be themed