feat(message-reader): add css variable name provider

Author: rafaeltonholo

feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssVariableNameProvider.kt

@@ -0,0 +1,14 @@
+package net.thunderbird.feature.mail.message.reader.api.css
+
+/**
+ * Provides CSS variable values for theming the message viewer.
+ *
+ * Implementations of this interface supply the name of the variables (e.g., "--my-variable")
+ * that will be used in the message display's stylesheet.
+ */
+interface CssVariableNameProvider {
+    /**
+     * The name of the CSS variable used to specify the 'border-left-color' for blockquote elements.
+     */
+    val blockquoteDefaultBorderLeftColor: String
+}

feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep

@@ -0,0 +1,11 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+
+internal class DefaultCssVariableNameProvider(
+    cssClassNameProvider: CssClassNameProvider,
+) : CssVariableNameProvider {
+    override val blockquoteDefaultBorderLeftColor: String =
+        "--${cssClassNameProvider.defaultNamespaceClassName}__blockquote-default-border-color"
+}