help: Add white background to icons for dark mode

help/C/figures/help-about-symbolic.svg

@@ -1,4 +1,5 @@
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+  <defs/>
+  <path id="rect1" fill="#ffffff" d="M1.368,0 L18.632,0 A1.368 1.368 0 0 1 20,1.368 L20,18.632 A1.368 1.368 0 0 1 18.632,20 L1.368,20 A1.368 1.368 0 0 1 0,18.632 L0,1.368 A1.368 1.368 0 0 1 1.368,0 Z"/>
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