src/vs/workbench/services/themes/browser/fileIconThemeData.ts/resolvePath
function _processIconThemeDocument(id: string, iconThemeDocumentLocation: URI, iconThemeDocument: IconThemeDocument): { content: string; hasFileIcons: boolean; hasFolderIcons: boolean; hidesExplorerArrows: boolean; } {

	const result = { content: '', hasFileIcons: false, hasFolderIcons: false, hidesExplorerArrows: !!iconThemeDocument.hidesExplorerArrows };

	if (!iconThemeDocument.iconDefinitions) {
		return result;
	}
	let selectorByDefinitionId: { [def: string]: string[] } = {};

	const iconThemeDocumentLocationDirname = resources.dirname(iconThemeDocumentLocation);
	function resolvePath(path: string) {
		return resources.joinPath(iconThemeDocumentLocationDirname, path);
	}

	function collectSelectors(associations: IconsAssociation | undefined, baseThemeClassName?: string) {
		function addSelector(selector: string, defId: string) {
			if (defId) {
				let list = selectorByDefinitionId[defId];
				if (!list) {
					list = selectorByDefinitionId[defId] = [];
				}
				list.push(selector);
			}
		}
		if (associations) {
			let qualifier = '.show-file-icons';
			if (baseThemeClassName) {
				qualifier = baseThemeClassName + ' ' + qualifier;
			}

			const expanded = '.monaco-tl-twistie.collapsible:not(.collapsed) + .monaco-tl-contents';

			if (associations.folder) {
				addSelector(`${qualifier} .folder-icon::before`, associations.folder);
				result.hasFolderIcons = true;
			}

			if (associations.folderExpanded) {
				addSelector(`${qualifier} ${expanded} .folder-icon::before`, associations.folderExpanded);
				result.hasFolderIcons = true;
			}

			let rootFolder = associations.rootFolder || associations.folder;
			let rootFolderExpanded = associations.rootFolderExpanded || associations.folderExpanded;

			if (rootFolder) {
				addSelector(`${qualifier} .rootfolder-icon::before`, rootFolder);
				result.hasFolderIcons = true;
			}

			if (rootFolderExpanded) {
				addSelector(`${qualifier} ${expanded} .rootfolder-icon::before`, rootFolderExpanded);
				result.hasFolderIcons = true;
			}

			if (associations.file) {
				addSelector(`${qualifier} .file-icon::before`, associations.file);
				result.hasFileIcons = true;
			}

			let folderNames = associations.folderNames;
			if (folderNames) {
				for (let folderName in folderNames) {
					addSelector(`${qualifier} .${escapeCSS(folderName.toLowerCase())}-name-folder-icon.folder-icon::before`, folderNames[folderName]);
					result.hasFolderIcons = true;
				}
			}
			let folderNamesExpanded = associations.folderNamesExpanded;
			if (folderNamesExpanded) {
				for (let folderName in folderNamesExpanded) {
					addSelector(`${qualifier} ${expanded} .${escapeCSS(folderName.toLowerCase())}-name-folder-icon.folder-icon::before`, folderNamesExpanded[folderName]);
					result.hasFolderIcons = true;
				}
			}

			let languageIds = associations.languageIds;
			if (languageIds) {
				if (!languageIds.jsonc && languageIds.json) {
					languageIds.jsonc = languageIds.json;
				}
				for (let languageId in languageIds) {
					addSelector(`${qualifier} .${escapeCSS(languageId)}-lang-file-icon.file-icon::before`, languageIds[languageId]);
					result.hasFileIcons = true;
				}
			}
			let fileExtensions = associations.fileExtensions;
			if (fileExtensions) {
				for (let fileExtension in fileExtensions) {
					let selectors: string[] = [];
					let segments = fileExtension.toLowerCase().split('.');
					if (segments.length) {
						for (let i = 0; i < segments.length; i++) {
							selectors.push(`.${escapeCSS(segments.slice(i).join('.'))}-ext-file-icon`);
						}
						selectors.push('.ext-file-icon'); // extra segment to increase file-ext score
					}
					addSelector(`${qualifier} ${selectors.join('')}.file-icon::before`, fileExtensions[fileExtension]);
					result.hasFileIcons = true;
				}
			}
			let fileNames = associations.fileNames;
			if (fileNames) {
				for (let fileName in fileNames) {
					let selectors: string[] = [];
					fileName = fileName.toLowerCase();
					selectors.push(`.${escapeCSS(fileName)}-name-file-icon`);
					let segments = fileName.split('.');
					if (segments.length) {
						for (let i = 1; i < segments.length; i++) {
							selectors.push(`.${escapeCSS(segments.slice(i).join('.'))}-ext-file-icon`);
						}
						selectors.push('.ext-file-icon'); // extra segment to increase file-ext score
					}
					addSelector(`${qualifier} ${selectors.join('')}.file-icon::before`, fileNames[fileName]);
					result.hasFileIcons = true;
				}
			}
		}
	}
	collectSelectors(iconThemeDocument);
	collectSelectors(iconThemeDocument.light, '.vs');
	collectSelectors(iconThemeDocument.highContrast, '.hc-black');

	if (!result.hasFileIcons && !result.hasFolderIcons) {
		return result;
	}

	let cssRules: string[] = [];

	let fonts = iconThemeDocument.fonts;
	if (Array.isArray(fonts)) {
		fonts.forEach(font => {
			let src = font.src.map(l => `${asCSSUrl(resolvePath(l.path))} format('${l.format}')`).join(', ');
			cssRules.push(`@font-face { src: ${src}; font-family: '${font.id}'; font-weight: ${font.weight}; font-style: ${font.style}; }`);
		});
		cssRules.push(`.show-file-icons .file-icon::before, .show-file-icons .folder-icon::before, .show-file-icons .rootfolder-icon::before { font-family: '${fonts[0].id}'; font-size: ${fonts[0].size || '150%'}}`);
	}

	for (let defId in selectorByDefinitionId) {
		let selectors = selectorByDefinitionId[defId];
		let definition = iconThemeDocument.iconDefinitions[defId];
		if (definition) {
			if (definition.iconPath) {
				cssRules.push(`${selectors.join(', ')} { content: ' '; background-image: ${asCSSUrl(resolvePath(definition.iconPath))}; }`);
			}
			if (definition.fontCharacter || definition.fontColor) {
				let body = '';
				if (definition.fontColor) {
					body += ` color: ${definition.fontColor};`;
				}
				if (definition.fontCharacter) {
					body += ` content: '${definition.fontCharacter}';`;
				}
				if (definition.fontSize) {
					body += ` font-size: ${definition.fontSize};`;
				}
				if (definition.fontId) {
					body += ` font-family: ${definition.fontId};`;
				}
				cssRules.push(`${selectors.join(', ')} { ${body} }`);
			}
		}
	}
	result.content = cssRules.join('\n');
	return result;
}