Replace Less with CSS (#23481)

Ran most of the Less files through the Less compiler and Prettier and
then followed up with a round of manual fixes.

The Less compiler had unfortunately stripped all `//` style comments
that I had to restore (It did preserve `/* */` comments). Other fixes
include duplicate selector removal which were revealed after the
transpilation and which weren't caught by stylelint before but now are.

Fixes: #15565

Author: silverwind

Diff for: .stylelintrc.yaml

@@ -5,8 +5,6 @@ ignoreFiles:
   - "**/*.go"
 
 overrides:
-  - files: ["**/*.less"]
-    customSyntax: postcss-less
   - files: ["**/chroma/*", "**/codemirror/*", "**/standalone/*", "**/console/*"]
     rules:
       scale-unlimited/declaration-strict-value: null

Diff for: Makefile

@@ -105,7 +105,7 @@ GO_TEST_PACKAGES ?= $(filter-out $(shell $(GO) list code.gitea.io/gitea/models/m
 
 FOMANTIC_WORK_DIR := web_src/fomantic
 
-WEBPACK_SOURCES := $(shell find web_src/js web_src/less -type f)
+WEBPACK_SOURCES := $(shell find web_src/js web_src/css -type f)
 WEBPACK_CONFIGS := webpack.config.js
 WEBPACK_DEST := public/js/index.js public/css/index.css
 WEBPACK_DEST_ENTRIES := public/js public/css public/fonts public/img/webpack public/serviceworker.js
@@ -131,7 +131,7 @@ TEST_TAGS ?= sqlite sqlite_unlock_notify
 TAR_EXCLUDES := .git data indexers queues log node_modules $(EXECUTABLE) $(FOMANTIC_WORK_DIR)/node_modules $(DIST) $(MAKE_EVIDENCE_DIR) $(AIR_TMP_DIR) $(GO_LICENSE_TMP_DIR)
 
 GO_DIRS := cmd tests models modules routers build services tools
-WEB_DIRS := web_src/js web_src/less
+WEB_DIRS := web_src/js web_src/css
 
 GO_SOURCES := $(wildcard *.go)
 GO_SOURCES += $(shell find $(GO_DIRS) -type f -name "*.go" -not -path modules/options/bindata.go -not -path modules/public/bindata.go -not -path modules/templates/bindata.go)
@@ -342,7 +342,7 @@ lint: lint-frontend lint-backend
 .PHONY: lint-frontend
 lint-frontend: node_modules
 	npx eslint --color --max-warnings=0 --ext js,vue web_src/js build *.config.js docs/assets/js tests/e2e
-	npx stylelint --color --max-warnings=0 web_src/less
+	npx stylelint --color --max-warnings=0 web_src/css
 	npx spectral lint -q -F hint $(SWAGGER_SPEC)
 	npx markdownlint docs *.md
 

Diff for: docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -354,7 +354,7 @@ To make a custom theme available to all users:
 
 Community themes are listed in [gitea/awesome-gitea#themes](https://gitea.com/gitea/awesome-gitea#themes).
 
-The `arc-green` theme source can be found [here](https://github.com/go-gitea/gitea/blob/main/web_src/less/themes/theme-arc-green.less).
+The `arc-green` theme source can be found [here](https://github.com/go-gitea/gitea/blob/main/web_src/css/themes/theme-arc-green.css).
 
 If your custom theme is considered a dark theme, set the global css variable `--is-dark-theme` to `true`.
 This allows Gitea to adjust the Monaco code editor's theme accordingly.

Diff for: docs/content/doc/developers/guidelines-frontend.en-us.md

@@ -21,13 +21,13 @@ menu:
 
 ## Background
 
-Gitea uses [Less CSS](https://lesscss.org), [Fomantic-UI](https://fomantic-ui.com/introduction/getting-started.html) (based on [jQuery](https://api.jquery.com)) and [Vue3](https://vuejs.org/) for its frontend.
+Gitea uses [Fomantic-UI](https://fomantic-ui.com/introduction/getting-started.html) (based on [jQuery](https://api.jquery.com)) and [Vue3](https://vuejs.org/) for its frontend.
 
 The HTML pages are rendered by [Go HTML Template](https://pkg.go.dev/html/template).
 
 The source files can be found in the following directories:
 
-* **Less styles:** `web_src/less/`
+* **CSS styles:** `web_src/css/`
 * **JavaScript files:** `web_src/js/`
 * **Vue components:** `web_src/js/components/`
 * **Go HTML templates:** `templates/`