[ui][python console] Fix hard to read python console editor icons

[nirvn]

Description

This PR fixes #59772 by altering two icons to insure their visibility is OK on both light / gray / dark themes.

Blend of gray before:

Blend of gray after:

Night mapping before:

Night mapping after:

[github-actions]

🪟 Windows builds

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Debug symbols for this build are available here.
(Built from commit 187168a)

🪟 Windows Qt6 builds

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(Built from commit 187168a)

[nyalldawson]

Instead of the buffer (which is a little ugly), how about using parameterised colors here? QgsApplication::getThemeIcon supports that already, so we could keep the single-color icon but render it using a suitable color based on the palette

[nirvn]

@nyalldawson , done.

The ugly buffer is something we rely on for quite a few icons, eg:

We can fix that one day :)