diff --git a/clean_files.txt b/clean_files.txt
index 3b24f975..fd051078 100644
--- a/clean_files.txt
+++ b/clean_files.txt
@@ -47,6 +47,7 @@ themes/bobby-python
 themes/brainy
 themes/brunton
 themes/command_duration.theme.bash
+themes/modern
 
 # plugins
 #
diff --git a/themes/modern/modern.theme.bash b/themes/modern/modern.theme.bash
index cfc20e9a..f14fd4fd 100644
--- a/themes/modern/modern.theme.bash
+++ b/themes/modern/modern.theme.bash
@@ -1,3 +1,5 @@
+# shellcheck shell=bash
+
 SCM_THEME_PROMPT_PREFIX=""
 SCM_THEME_PROMPT_SUFFIX=""
 
@@ -19,14 +21,14 @@ esac
 PS3=">> "
 
 is_vim_shell() {
-	if [ ! -z "$VIMRUNTIME" ]; then
+	if [ -n "$VIMRUNTIME" ]; then
 		echo "[${cyan}vim shell${normal}]"
 	fi
 }
 
 modern_scm_prompt() {
 	CHAR=$(scm_char)
-	if [ $CHAR = $SCM_NONE_CHAR ]; then
+	if [ "$CHAR" = "$SCM_NONE_CHAR" ]; then
 		return
 	else
 		echo "[$(scm_char)][$(scm_prompt_info)]"
@@ -44,7 +46,8 @@ detect_venv() {
 }
 
 prompt() {
-	if [ $? -ne 0 ]; then
+	retval=$?
+	if [[ retval -ne 0 ]]; then
 		PS1="${TITLEBAR}${bold_red}┌─${reset_color}$(modern_scm_prompt)[${cyan}\u${normal}][${cyan}\w${normal}]$(is_vim_shell)\n${bold_red}└─▪${normal} "
 	else
 		PS1="${TITLEBAR}┌─$(modern_scm_prompt)[${cyan}\u${normal}][${cyan}\w${normal}]$(is_vim_shell)\n└─▪ "