Replace compiled Less colors with CSS var() function calls

library/Icinga/Less/ColorProp.php

@@ -0,0 +1,99 @@
+<?php
+
+namespace Icinga\Less;
+
+use Less_Tree_Call;
+use Less_Tree_Color;
+use Less_Tree_Keyword;
+
+class ColorProp extends Less_Tree_Color
+{
+    /** @var Less_Tree_Color */
+    protected $color;
+
+    /** @var int */
+    protected $index;
+
+    /** @var string */
+    protected $origin;
+
+    public function __construct()
+    {
+    }
+
+    /**
+     * @param Less_Tree_Color $color
+     *
+     * @return self
+     */
+    public static function fromColor(Less_Tree_Color $color)
+    {
+        $self = new self();
+        $self->color = $color;
+
+        foreach ($color as $k => $v) {
+            $self->$k = $v;
+        }
+
+        return $self;
+    }
+
+    /**
+     * @return int
+     */
+    public function getIndex()
+    {
+        return $this->index;
+    }
+
+    /**
+     * @param int $index
+     *
+     * @return $this
+     */
+    public function setIndex($index)
+    {
+        $this->index = $index;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getOrigin()
+    {
+        return $this->origin;
+    }
+
+    /**
+     * @param string $origin
+     *
+     * @return $this
+     */
+    public function setOrigin($origin)
+    {
+        $this->origin = $origin;
+
+        return $this;
+    }
+
+    public function compile()
+    {
+        return $this;
+    }
+
+    public function genCSS($output)
+    {
+        $css = (new Less_Tree_Call(
+            'var',
+            [
+                new Less_Tree_Keyword('--' . $this->getOrigin()),
+                $this->color
+            ],
+            $this->getIndex()
+        ))->toCSS();
+
+        $output->add($css);
+    }
+}

library/Icinga/Less/ColorPropOrVariable.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Icinga\Less;
+
+use Less_Tree;
+use Less_Tree_Color;
+use Less_Tree_Variable;
+
+class ColorPropOrVariable extends Less_Tree
+{
+    public $type = 'Variable';
+
+    /** @var Less_Tree_Variable */
+    protected $variable;
+
+    /**
+     * @return Less_Tree_Variable
+     */
+    public function getVariable()
+    {
+        return $this->variable;
+    }
+
+    /**
+     * @param Less_Tree_Variable $variable
+     *
+     * @return $this
+     */
+    public function setVariable(Less_Tree_Variable $variable)
+    {
+        $this->variable = $variable;
+
+        return $this;
+    }
+
+    public function compile($env)
+    {
+        $v = $this->getVariable();
+        $compiled = $v->compile($env);
+
+        if ($compiled instanceof Less_Tree_Color) {
+            return ColorProp::fromColor($compiled)
+                ->setIndex($v->index)
+                ->setOrigin(substr($v->name, 1));
+        }
+
+        return $compiled;
+    }
+}

library/Icinga/Less/Visitor.php

@@ -0,0 +1,96 @@
+<?php
+
+namespace Icinga\Less;
+
+use Less_VisitorReplacing;
+use LogicException;
+
+/**
+ * Replace compiled Less colors with CSS var() function calls
+ */
+class Visitor extends Less_VisitorReplacing
+{
+    public $isPreEvalVisitor = true;
+
+    /**
+     * Whether calling var() CSS function
+     *
+     * If that's the case, don't try to replace compiled Less colors with CSS var() function calls.
+     *
+     * @var bool|string
+     */
+    protected $callingVar = false;
+
+    /**
+     * Whether defining a variable
+     *
+     * If that's the case, don't try to replace compiled Less colors with CSS var() function calls.
+     *
+     * @var bool|string
+     */
+    protected $definingVar = false;
+
+    public function visitCall($c)
+    {
+        if ($c->name === 'var') {
+            if ($this->callingVar !== false) {
+                throw new LogicException('Already calling var');
+            }
+
+            $this->callingVar = spl_object_hash($c);
+        }
+
+        return $c;
+    }
+
+    public function visitCallOut($c)
+    {
+        if ($this->callingVar !== false && $this->callingVar === spl_object_hash($c)) {
+            $this->callingVar = false;
+        }
+    }
+
+    public function visitDetachedRuleset($rs)
+    {
+        // A detached ruleset is a variable definition in the first place,
+        // so just reset that we define a variable.
+        $this->definingVar = false;
+
+        return $rs;
+    }
+
+    public function visitRule($r)
+    {
+        if ($r->name[0] === '@' && $r->variable) {
+            if ($this->definingVar !== false) {
+                throw new LogicException('Already defining a variable');
+            }
+
+            $this->definingVar = spl_object_hash($r);
+        }
+
+        return $r;
+    }
+
+    public function visitRuleOut($r)
+    {
+        if ($this->definingVar !== false && $this->definingVar === spl_object_hash($r)) {
+            $this->definingVar = false;
+        }
+    }
+
+    public function visitVariable($v)
+    {
+        if ($this->callingVar !== false || $this->definingVar !== false) {
+            return $v;
+        }
+
+        return (new ColorPropOrVariable())
+            ->setVariable($v);
+    }
+
+    public function run($node)
+    {
+        return $this->visitObj($node);
+    }
+}

library/Icinga/Util/LessParser.php

@@ -3,6 +3,7 @@
 
 namespace Icinga\Util;
 
+use Icinga\Less\Visitor;
 use Less_Tree_Anonymous;
 use Less_Tree_Expression;
 use Less_Tree_Quoted;
@@ -16,6 +17,7 @@ class LessParser extends lessc
     public function __construct()
     {
         $this->registerFunction('extract-variable-default', [$this, 'extractVariableDefault']);
+        $this->setOption('plugins', [new Visitor()]);
     }
 
     /**