::First let me clear that you can't edit magento core themes directly from /app/design/magento!!!::
::If you want to change it your self then, you've to create your own one and make changes as required!!::
::Now let's go to the point!::
::first => Create custom.css file at /app/design/vendor/theme/Magento_Theme/web/css/::
::second => create default.xml file at /app/design/vendor/theme/Magento_Theme/layout/::
::Now put the file contents like below::
::CSS::
::
.minicart-wrapper .action.showcart:before{
content:url('../images/minicart.jpg')!important;
zoom:1.5%;}
::
::XML::
<head>
<css src="Magento_Theme::css/custom.css"/>
</head>
:::Last Step :::
move your new minicart image to the /app/design/vendor/theme/Magento_Theme/web/images/minicart.img extension
::Finally run below commands::
::Make sure to remove /pub/static/frontend::
::bin/magento setup:upgrade::
::bin/magento setup:di:compile::
::bin/magento setup:static-content:deploy -f::
::bin/magento indexer:reindex::
::bin/magento cache:flush::
::chmod -R 777 ../magento root directory name::
::Thank You::