Explanation
The favicon can be added to the <your_theme_dir>/Magento_Theme/web/ directory of a custom theme. The favicon should have the name favicon.ico and replace the default one from the Magento_Theme module. The
<your_theme_dir>/web/ directory is used for storing web assets that are not specific to any module. The
<your_theme_dir>/assets/images directory does not exist by default and is not used for storing favicons.
References: [Add a favicon], [Theme structure]

Explanation
To customize the header of email templates, the developer needs to place the new template in the
/Magento_Email/email/header.html path of the theme. This will override the default header template from the Magento_Email module. The /Magento_Email/templates/override/html/header.html path is not valid and will not work. The /Magento_Theme/html/header.html path is used for customizing the header of web pages, not emails. References: [Customize email templates], [Email templates overview]

Explanation
To delete a manually installed theme, the developer needs to remove the theme directory from the app/design/frontend directory and also delete the corresponding record from the theme table in the database.
The theme:uninstall CLI command is only used for deleting themes thatare installed as Composer packages.
Disabling the theme from the backend admin configuration does not delete the theme files or records, but only makes it unavailable for use. References: [Delete a theme], [theme:uninstall]

Explanation
To implement a custom font specifically for emails, the developer needs to do the following steps:
Add the custom font file to the web/fonts directory of the custom theme.
Use the @import font function with the url of the custom font from the theme in the
/Vendor/Theme/web/css/source/_extend.less file. This will import the custom font and make it available for use in other LESS files. For example:
@import font('custom-font', '@{baseDir}fonts/custom-font.ttf', 'truetype'); Add in the styles to target the elements that require being changed in the
/Vendor/Theme/web/css/source/_email.less file. This file is used to define the styles for email templates.
The developer can use the .lib-font-face() mixin to apply the custom font to specific selectors. For example:
lib-font-face( @family-name: @custom-font, @font-path: '@{baseDir}fonts/custom-font', @font-weight:
normal, @font-style: normal );
h1 { .lib-font-face( @family-name: @custom-font, @font-path: '@{baseDir}fonts/custom-font',
@font-weight: normal, @font-style: normal ); }
The /vendor/Theme/web/css/source/_typography.less file is not suitable for implementing a custom font for emails, as it is used for defining global typography styles for web pages. The <head></head> tag is not used for adding fonts in email templates, as it is not supported by most email clients. References: [Custom fonts],
[Email templates overview]

Explanation
To register a new page layout in a custom theme, the developer needs to create a layouts.xml file in the app/design/frontend/<VendorName>/<ThemeName>/Magento_Theme/layout directory. The layouts.xml file should contain the <layout> element with the id, label, and file attributes. The id attribute is used to reference the layout in other layout files, the label attribute is used to display the layout name in the admin panel, and the file attribute is used to specify the path to the layout file relative to the web directory of the theme. The app/design/frontend/<VendorName>/<ThemeName>/layouts.xml and app/design/frontend/<VendorName>/<ThemeName>/Magento_Theme/layouts.xml files are not valid and will not work. References: [Create a new page layout], [layouts.xml]