diff --git a/css/dialogs.css b/css/dialogs.css
index 334b8bf4..e0bdf64a 100644
--- a/css/dialogs.css
+++ b/css/dialogs.css
@@ -804,6 +804,7 @@
 		background-color: var(--color-frame);
 		border: 2px solid var(--color-frame);
 		border-top: none;
+		overflow: hidden;
 	}
 	.custom_thumbnail_preview {
 		position: relative;
diff --git a/js/interface/themes.js b/js/interface/themes.js
index f2635be9..6b8b5b84 100644
--- a/js/interface/themes.js
+++ b/js/interface/themes.js
@@ -442,15 +442,21 @@ const CustomTheme = {
 			style.textContent = theme.thumbnail;
 			const sheet = style.sheet;
 			for (const rule of sheet.cssRules) {
-		    thumbnailStyles += `.custom_theme_thumbnail_${theme.id} ${rule.selectorText} { ${rule.style.cssText} }\n`;
-		  }
+				const selectors = rule.selectorText.split(/(?<!\[[^\]]*),(?![^\[]*\])|(?<!"[^"]*),(?![^"]*")/g).map(e => e.trim())
+				for (const selector of selectors) {
+					thumbnailStyles += `.custom_theme_thumbnail_${theme.id} ${selector} { ${rule.style.cssText} }\n`;
+				}
+			}
 		}
 		if (CustomTheme.data.customized) {
 			style.textContent = CustomTheme.data.thumbnail;
 			const sheet = style.sheet;
 			for (const rule of sheet.cssRules) {
-		    thumbnailStyles += `.custom_theme_thumbnail_${CustomTheme.data.id} ${rule.selectorText} { ${rule.style.cssText} }\n`;
-		  }
+				const selectors = rule.selectorText.split(/(?<!\[[^\]]*),(?![^\[]*\])|(?<!"[^"]*),(?![^"]*")/g).map(e => e.trim())
+				for (const selector of selectors) {
+					thumbnailStyles += `.custom_theme_thumbnail_${CustomTheme.data.id} ${selector} { ${rule.style.cssText} }\n`;
+				}
+			}
 		}
 		document.head.removeChild(style);
 		$('style#theme_thumbnail_css').text(thumbnailStyles);