Make captcha more readable

Especially on dark theme

Signed-off-by: Minecon724 <minecon724@noreply.git.m724.eu>

web_src/css/form.css

@@ -334,6 +334,10 @@ input:-webkit-autofill:active,
   display: inline-block;
 }
 
+.captcha-img {
+  width: min(30em, 100%);
+}
+
 @media (min-width: 768px) {
   .g-recaptcha-style,
   .h-captcha-style {

web_src/css/markup/dark.css

@@ -11,3 +11,10 @@
 .markup [href$="#dark-mode-only"] {
   display: unset;
 }
+
+
+/* The reason we're doing this *here* is because dark vanilla templates always use this file, and there's no other way to detect dark theme */
+
+.captcha-img {
+  filter: invert();
+}