fix(theme): problems with theme preview (@fehmer) (monkeytypegame#6881)

1. when a custom theme is active preview of regular theme was not
applied, introduced with
monkeytypegame@fdead53
2. when custom theme is active previewing other custom themes set the
color to the active custom theme first, then the selected one causing
flashing

To reproduce:

1. preview of regular theme not working 
- create a custom theme with black background, activate it
- open commandline to preview a regular theme

2. custom theme colors get set multiple times
- create a custom theme with white background called white and activate
it
- create to custom themes with black background called black1 and black2
- switch preview between black1 and black2

Author: fehmer

frontend/src/ts/commandline/commandline.ts

@@ -556,7 +556,7 @@ async function updateActiveCommand(): Promise<void> {
   activeCommand = command ?? null;
   if (element === undefined || command === undefined) {
     clearFontPreview();
-    void ThemeController.clearPreview();
+    void ThemeController.clearPreview(false);
     addCommandlineBackground();
     return;
   }
@@ -567,7 +567,7 @@ async function updateActiveCommand(): Promise<void> {
   if (/changeTheme.+/gi.test(command.id)) {
     removeCommandlineBackground();
   } else {
-    void ThemeController.clearPreview();
+    void ThemeController.clearPreview(false);
     addCommandlineBackground();
   }
 

frontend/src/ts/controllers/theme-controller.ts

@@ -154,7 +154,8 @@ async function apply(
     customColorsOverride,
     isPreview
   );
-  if (!Config.customTheme) {
+
+  if (themeName !== "custom") {
     clearCustomTheme();
   }
   const name = customColorsOverride ? "custom" : themeName;