Fix loading new themes from face-remapped buffers

E.g. If mixed-pitch-mode is enabled and you change themes, then open a
new frame, the theme is messed up.

Also refactor load-theme advice into one.

core/core-ui.el

@@ -598,20 +598,25 @@ behavior). Do not set this directly, this is let-bound in `doom-init-theme-h'.")
       (let ((doom--prefer-theme-elc t)) ; DEPRECATED in Emacs 27
         (load-theme doom-theme t)))))
 
-(defadvice! doom--run-load-theme-hooks-a (theme &optional _no-confirm no-enable)
-  "Set up `doom-load-theme-hook' to run after `load-theme' is called."
-  :after-while #'load-theme
-  (unless no-enable
-    (setq doom-theme theme
-          doom-init-theme-p t)
-    (run-hooks 'doom-load-theme-hook)))
+(defadvice! doom--load-theme-a (orig-fn theme &optional no-confirm no-enable)
+  "Run `doom-load-theme-hook' on `load-theme' and fix its issues.
 
-(defadvice! doom--disable-enabled-themes-a (theme &optional _no-confirm no-enable)
-  "Disable previously enabled themes before loading a new one.
-Otherwise, themes can conflict with each other."
-  :after-while #'load-theme
-  (unless no-enable
-    (mapc #'disable-theme (remq theme custom-enabled-themes))))
+1. Disable previously enabled themes.
+2. Don't let face-remapping screw up loading the new theme
+   (*cough*`mixed-pitch-mode').
+3. Record the current theme in `doom-theme'."
+  :around #'load-theme
+  ;; HACK Run `load-theme' from an estranged buffer, where we can be assured
+  ;;      that buffer-local face remaps (by `mixed-pitch-mode', for instance)
+  ;;      won't interfere with changing themes.
+  (with-temp-buffer
+    (when-let (result (funcall orig-fn theme no-confirm no-enable))
+      (unless no-enable
+        (setq doom-theme theme
+              doom-init-theme-p t)
+        (mapc #'disable-theme (remq theme custom-enabled-themes))
+        (run-hooks 'doom-load-theme-hook))
+      result)))
 
 (unless EMACS27+
   ;; DEPRECATED Not needed in Emacs 27