Markup color and font size fixes (#30282) (#30310)

Backport #30282 by @silverwind

1. Distinguish inline an block code with new CSS variable
`--color-markup-code-inline`
2. Various color tweaks, better contrast from background

<img width="447" alt="Screenshot 2024-04-05 at 00 51 00"
src="https://github.com/go-gitea/gitea/assets/115237/93e069f4-6807-4f2c-9331-2d69730919d4">
<img width="456" alt="Screenshot 2024-04-05 at 00 50 44"
src="https://github.com/go-gitea/gitea/assets/115237/0dc9c745-c531-40fa-94ec-b0ba10bd7ccf">

Co-authored-by: silverwind <me@silverwind.io>

Author: GiteaBot

web_src/css/markup/content.css

@@ -438,7 +438,7 @@
   margin: 0;
   font-size: 85%;
   white-space: break-spaces;
-  background-color: var(--color-markup-code-block);
+  background-color: var(--color-markup-code-inline);
   border-radius: var(--border-radius);
 }
 
@@ -508,7 +508,7 @@
   line-height: 10px;
   color: var(--color-text-light);
   vertical-align: middle;
-  background-color: var(--color-markup-code-block);
+  background-color: var(--color-markup-code-inline);
   border: 1px solid var(--color-secondary);
   border-radius: var(--border-radius);
   box-shadow: inset 0 -1px 0 var(--color-secondary);

web_src/css/themes/theme-gitea-dark.css

@@ -204,8 +204,9 @@
   --color-active: #e8e8ff24;
   --color-menu: #151a1e;
   --color-card: #151a1e;
-  --color-markup-table-row: #e8e8ff06;
-  --color-markup-code-block: #e8e8ff16;
+  --color-markup-table-row: #e8e8ff0f;
+  --color-markup-code-block: #e8e8ff12;
+  --color-markup-code-inline: #e8e8ff28;
   --color-button: #151a1e;
   --color-code-bg: #14171a;
   --color-shadow: #00001758;

web_src/css/themes/theme-gitea-light.css

@@ -204,8 +204,9 @@
   --color-active: #00001714;
   --color-menu: #f8f9fb;
   --color-card: #f8f9fb;
-  --color-markup-table-row: #00001708;
-  --color-markup-code-block: #00001710;
+  --color-markup-table-row: #0030600a;
+  --color-markup-code-block: #00306010;
+  --color-markup-code-inline: #00306012;
   --color-button: #f8f9fb;
   --color-code-bg: #fafdff;
   --color-shadow: #00001726;