MDL-83933 theme: Update default icon width

MDL-83933 theme: Update default icon width

While updating the custom icons for import/export, it was observed
that the default width for FontAwesome icons is 20px instead of
the previously assumed 16px.
This commit adjusts the default width accordingly and resolves several
styles where the width was incorrectly applied.
Additionally, the default icon color has been updated to ensure consistent
appearance across both SVG icons and FontAwesome icons.