[bugfix/frontend] Fix text-shadow on dark mode themes (#4216)

Just deals with a few remaining straggler bits of css fuckery that I introduced in https://codeberg.org/superseriousbusiness/gotosocial/pulls/4201

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4216
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

web/assets/themes/ecks-pee.css

@@ -3,6 +3,9 @@
   theme-description: Looks suspiciously like a certain operating system
 */
 
+/* Use dark code highlights. */
+@import url("../dist/_prism-dark.css");
+
 :root {
   /* Define our color palette */
   --ecks-pee-blurple: #3833ac;

web/assets/themes/midnight-trip.css

@@ -3,6 +3,9 @@
   theme-description: Woah
 */
 
+/* Use dark code highlights. */
+@import url("../dist/_prism-dark.css");
+
 /* Theme colors */
 :root {
   --acid-green: rgb(63, 255, 0);

web/assets/themes/moonlight-hunt.css

@@ -3,6 +3,9 @@
   theme-description: Ominous dark blue / black with a tinge of blood red. You may think it all a mere bad dream.
 */
 
+/* Use dark code highlights. */
+@import url("../dist/_prism-dark.css");
+
 :root {
   /* Define our palette */
   --bleached-bone: #f3e3d4;

web/assets/themes/rain-forest.css

@@ -7,6 +7,9 @@
    color names as reference
    v1.0 by xmgz at github */
 
+/* Use dark code highlights. */
+@import url("../dist/_prism-dark.css");
+
 :root {
   /* color definitions */
   --dgreen1: #003333;