Does the double-selector work for both GTK+ versions?

That's a good question. I would imagine it would work on < 3.20 too, but I only tested on 3.20.3 for the moment.

Also, why the margin: -2px; is needed at all?

Adwaita has the equivalent to margin: 4px 4px 4px -4px; on the button, which makes it take a lot of space, and be too close to the left element. The idea with -2 was to not take space for the button's borders alike, prefering having it possibly overflow 1 pixel when hovered than taking more place.
But that was a great question as it was actually mostly a hack, and you made me investigate further why it was larger than I believed was necessary, and I found out that Adwaita has a min-width and min-height of about 20 (IIRC) on the button. Resetting that makes the button actually smaller, removing any use for the negative margin.

$ git diff
diff --git a/data/geany-3.20.css b/data/geany-3.20.css
index 8388818..5cbde30 100644
--- a/data/geany-3.20.css
+++ b/data/geany-3.20.css
@@ -4,5 +4,8 @@
 #geany-close-tab-button {
     outline-offset: 0;
     outline-width: 0;
-    border: 0;
+    margin: 0;
+    margin-left: 0.5em;
+    min-width: 0;
+    min-height: 0;
 }
diff --git a/data/geany.css b/data/geany.css
index 2fb7964..5b7f322 100644
--- a/data/geany.css
+++ b/data/geany.css
@@ -4,12 +4,14 @@
 #geany-close-tab-button {
     padding: 0;
 }
-#geany-close-tab-button GtkImage {
+#geany-close-tab-button GtkImage /* GTK < 3.20 */ ,
+#geany-close-tab-button image /* GTK >= 3.20 */ {
     padding: 0;
 }

 /* use monospaced font in search entries for easier reading of regexp (#1907117) */
-#GeanyDialogSearch GtkEntry {
+#GeanyDialogSearch GtkEntry /* GTK < 3.20 */,
+#GeanyDialogSearch entry /* GTK >= 3.20 */ {
     font-family: monospace;
 }

With this, I get a prettier small button that scales down very well:

The margin-left: 0.5em is not to get the button collapsed to the label, which I believe is prettier and more readable. Half an em seemed like a good visual spacing after a piece of text, but anything else would do.

I also played further if we wanna get creative and pretty, to make the buttons on non-active tabs lighter. It's easy enough with the (new?) CSS, but maybe we wanna make sure it's great with more than just Adwaita, although as I just played with the opacity it shouldn't make anything much less nice.

GEdit-ish, always light but on button hover:

#geany-close-tab-button {
    opacity: 0.25;
}
#geany-close-tab-button:hover {
    opacity: 1;
}

light-out only non-active tab, Adwaita-ish:

notebook tab #geany-close-tab-button {
    opacity: 0.25;
}
notebook tab:hover #geany-close-tab-button {
    opacity: 0.5;
}
notebook tab:checked #geany-close-tab-button,
notebook tab #geany-close-tab-button:hover {
    opacity: 1;
}

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