How do I determine which icon set to use for my application?

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Linux

UI

Delphi

GUI

Pascal

Bitmap

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Hello,

After installing a new distribution of GNU/Linux, I found that various applications were using a different icon set for their MenuStrips and ToolBars. My old distribution used the GTK+ icon set (Debian), my new distribution uses the Mint-X icon set (Linux Mint).

My question is: how does the application determine which Desktop Environment/distro is being used? Are there any easier ways of implementing the same thing without doing it from scratch, programmatically?

For my application, I am using Object Pascal (Lazarus); are there any libraries I can refer to to make this less of a headache?

Thanks you for your help and Happy New Year!

PS: Is it unprofessional to just use GTK+ icon sets across operating systems?

What I have tried:

Searching the web for more information, however none of my searches throw-up very useful information.