lib/preview: `shfmt` && `shellcheck`

Fix theme file path globbing when $BASH_IT contains any spaces.

My apologies to future `git blame` hunters ♥

clean_files.txt

@@ -80,6 +80,7 @@ completion/available/wpscan.completion.bash
 # libraries
 lib/helpers.bash
 lib/log.bash
+lib/preview.bash
 lib/search.bash
 lib/utilities.bash
 

lib/preview.bash

@@ -1,19 +1,20 @@
-if [[ "${BASH_PREVIEW:-}" ]];
-then
-  unset BASH_PREVIEW #Prevent infinite looping
-  echo "
+# shellcheck shell=bash
+#
+# Displays the prompt from each _Bash It_ theme.
+
+if [[ -n "${BASH_PREVIEW:-}" ]]; then
+	unset BASH_PREVIEW #Prevent infinite looping
+	echo "
 
   Previewing Bash-it Themes
 
   "
 
-  THEMES="$BASH_IT/themes/*/*.theme.bash"
-  for theme in $THEMES
-  do
-    BASH_IT_THEME=${theme%.theme.bash}
-    BASH_IT_THEME=${BASH_IT_THEME##*/}
-    echo "
+	for theme in "${BASH_IT?}/themes"/*/*.theme.bash; do
+		BASH_IT_THEME=${theme%.theme.bash}
+		BASH_IT_THEME=${BASH_IT_THEME##*/}
+		echo "
     $BASH_IT_THEME"
-    echo "" | bash --init-file "${BASH_IT}/bash_it.sh" -i
-  done
+		echo "" | bash --init-file "${BASH_IT?}/bash_it.sh" -i
+	done
 fi