Misc color tweaks (#29943)

Minor color tweaks:

- Better text contrasts
- Better distinguish nav and header wrapper in light theme
- Input boxes are now white on light theme
- Slightly darker dark theme background

<img width="503" alt="Screenshot 2024-03-20 at 19 31 54"
src="https://github.com/go-gitea/gitea/assets/115237/c7802a84-2386-4332-bd91-f419473ff644">
<img width="510" alt="Screenshot 2024-03-20 at 19 32 24"
src="https://github.com/go-gitea/gitea/assets/115237/21d3529e-6e0a-413a-9e9e-a03bea2405eb">

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

@@ -180,16 +180,16 @@
   --color-orange-badge-hover-bg: #f2711c4d;
   --color-git: #f05133;
   /* target-based colors */
-  --color-body: #1e2224;
+  --color-body: #1c1f25;
   --color-box-header: #1a1d1f;
   --color-box-body: #14171a;
   --color-box-body-highlight: #121517;
   --color-text-dark: #f8f8f9;
-  --color-text: #ced2d5;
-  --color-text-light: #bec4c8;
-  --color-text-light-1: #acb3b8;
-  --color-text-light-2: #8d969c;
-  --color-text-light-3: #747f87;
+  --color-text: #d1d5d8;
+  --color-text-light: #bdc3c7;
+  --color-text-light-1: #a8afb5;
+  --color-text-light-2: #929ba2;
+  --color-text-light-3: #7c8790;
   --color-footer: var(--color-nav-bg);
   --color-timeline: #353c42;
   --color-input-text: var(--color-text-dark);

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

@@ -184,20 +184,20 @@
   --color-box-header: #f1f3f5;
   --color-box-body: #ffffff;
   --color-box-body-highlight: #f4faff;
-  --color-text-dark: #03080d;
-  --color-text: #1c2126;
-  --color-text-light: #3c434a;
-  --color-text-light-1: #4b5259;
-  --color-text-light-2: #6a7178;
-  --color-text-light-3: #899097;
+  --color-text-dark: #01050a;
+  --color-text: #181c21;
+  --color-text-light: #30363b;
+  --color-text-light-1: #40474d;
+  --color-text-light-2: #5b6167;
+  --color-text-light-3: #747c84;
   --color-footer: var(--color-nav-bg);
   --color-timeline: #d0d7de;
   --color-input-text: var(--color-text-dark);
-  --color-input-background: #f8f9fb;
+  --color-input-background: #fff;
   --color-input-toggle-background: #d0d7de;
   --color-input-border: var(--color-secondary);
   --color-input-border-hover: var(--color-secondary-dark-1);
-  --color-header-wrapper: #fafbfc;
+  --color-header-wrapper: #f9fafb;
   --color-light: #00001706;
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(6 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #0000171d;
@@ -224,7 +224,7 @@
   --color-reaction-active-bg: var(--color-primary-light-6);
   --color-tooltip-text: #fbfdff;
   --color-tooltip-bg: #000017f0;
-  --color-nav-bg: #f8f9fb;
+  --color-nav-bg: #f6f7fa;
   --color-nav-hover-bg: var(--color-secondary-light-1);
   --color-nav-text: var(--color-text);
   --color-label-text: var(--color-text);