Support theme switching pre SDK 21

app/src/main/java/com/topjohnwu/magisk/ui/settings/SettingsViewModel.kt

@@ -52,11 +52,6 @@ class SettingsViewModel(
             Customization,
             Theme, Language
         )
-        if (Build.VERSION.SDK_INT < 21) {
-            // Pre 5.0 does not support getting colors from attributes,
-            // making theming a pain in the ass. Just forget about it
-            list.remove(Theme)
-        }
         if (isRunningAsStub && ShortcutManagerCompat.isRequestPinShortcutSupported(context))
             list.add(AddShortcut)
 

app/src/main/java/com/topjohnwu/magisk/ui/theme/ThemeFragment.kt

@@ -1,8 +1,16 @@
 package com.topjohnwu.magisk.ui.theme
 
+import android.os.Bundle
+import android.view.ContextThemeWrapper
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import android.widget.FrameLayout
+import com.topjohnwu.magisk.BR
 import com.topjohnwu.magisk.R
 import com.topjohnwu.magisk.arch.BaseUIFragment
 import com.topjohnwu.magisk.databinding.FragmentThemeMd2Binding
+import com.topjohnwu.magisk.databinding.ItemThemeBindingImpl
 import org.koin.androidx.viewmodel.ext.android.viewModel
 
 class ThemeFragment : BaseUIFragment<ThemeViewModel, FragmentThemeMd2Binding>() {
@@ -10,6 +18,46 @@ class ThemeFragment : BaseUIFragment<ThemeViewModel, FragmentThemeMd2Binding>()
     override val layoutRes = R.layout.fragment_theme_md2
     override val viewModel by viewModel<ThemeViewModel>()
 
+    private fun <T> Array<T>.paired(): List<Pair<T, T?>> {
+        val iterator = iterator()
+        if (!iterator.hasNext()) return emptyList()
+        val result = mutableListOf<Pair<T, T?>>()
+        while (iterator.hasNext()) {
+            val a = iterator.next()
+            val b = if (iterator.hasNext()) iterator.next() else null
+            result.add(a to b)
+        }
+        return result
+    }
+
+    override fun onCreateView(
+        inflater: LayoutInflater,
+        container: ViewGroup?,
+        savedInstanceState: Bundle?
+    ): View? {
+        super.onCreateView(inflater, container, savedInstanceState)
+
+        for ((a, b) in Theme.values().paired()) {
+            val c = inflater.inflate(R.layout.item_theme_container, null, false)
+            val left = c.findViewById<FrameLayout>(R.id.left)
+            val right = c.findViewById<FrameLayout>(R.id.right)
+
+            for ((theme, view) in listOf(a to left, b to right)) {
+                theme ?: continue
+                val themed = ContextThemeWrapper(activity, theme.themeRes)
+                ItemThemeBindingImpl.inflate(LayoutInflater.from(themed), view, true).also {
+                    it.setVariable(BR.viewModel, viewModel)
+                    it.setVariable(BR.theme, theme)
+                    it.lifecycleOwner = this
+                }
+            }
+
+            binding.themeContainer.addView(c)
+        }
+
+        return binding.root
+    }
+
     override fun onStart() {
         super.onStart()