[D8]: Setting default logo path in a theme

If a custom logo is not specified, Drupal will search in the active theme for a default logo image file. This has been changed from a PNG file to a SVG file.

Themes that were providing a default logo.png will need to provide a logo.svg. PNG logos, or other image formats, can always be used by uploading the logo as a custom logo.

Themers can create extra custom settings in their themes to give support to non-SVG default logos.

Consider that SVGs will not always have height and width dimensions, and that adding an SVG file with no dimensions could affect the styling of your theme.

To use non-svg file create a file in your theme in THEME_NAME/config/install/THEME_NAME.settings.yml and paste this code in it BEFORE installation(or re-install the theme so the config get's loaded):

  path: 'themes/THEME_NAME/logo.png'
  use_default: false

Reference: https://www.drupal.org/node/2410787

drupal 8themedevelopmentlogonon-svg