[A11Y] Text/Icon colors on colored backgrounds change in dark mode

## Bug Report

**Current Behavior**
Texts on colored background switched to dark text/icon color

**Steps to Reproduce**
1. Enable Dark Mode in admin settings
2. Enable "Colored header" and set color to "#5572F7"
3. Go back to main page

**Expected Behavior**
Texts on colored backgrounds continue to use a bright color.
Other web apps (Twitter, GitHub) also mostly use bright colors on their colored buttons

**Screenshots**

**Environment**
- Flarum version: 0.7.1
- Webserver: apache
- Hosting environment: shared
- PHP version: 7.4.15
- Browser: Edge Version 88.0.705.63

```
Flarum core 0.1.0-beta.15
PHP version: 7.4.15
Loaded extensions: Core, date, libxml, openssl, pcre, zlib, filter, hash, pcntl, readline, Reflection, SPL, session, standard, restrictdocroot, bcmath, bz2, calendar, ctype, curl, dba, dom, mbstring, fileinfo, ftp, gd, gettext, gmp, iconv, imap, intl, json, ldap, exif, mysqlnd, PDO, pgsql, Phar, posix, shmop, SimpleXML, soap, sockets, sodium, sqlite3, sysvmsg, sysvsem, sysvshm, tidy, tokenizer, xml, xmlwriter, xsl, mcrypt, mysqli, pdo_mysql, pdo_pgsql, pdo_sqlite, xmlreader, xmlrpc, apcu, decimal, gnupg, igbinary, imagick, msgpack, psr, yaml, zip, mongodb, phalcon, redis, Zend OPcache
+----------------------+------------------+--------+
| Flarum Extensions | | |
+----------------------+------------------+--------+
| ID | Version | Commit |
+----------------------+------------------+--------+
| flarum-approval | v0.1.0-beta.15 | |
| flarum-bbcode | v0.1.0-beta.15 | |
| flarum-emoji | v0.1.0-beta.15 | |
| flarum-lang-english | v0.1.0-beta.15 | |
| flarum-flags | v0.1.0-beta.15 | |
| flarum-lock | v0.1.0-beta.15 | |
| flarum-markdown | v0.1.0-beta.15 | |
| flarum-mentions | v0.1.0-beta.15 | |
| flarum-statistics | v0.1.0-beta.15 | |
| flarum-sticky | v0.1.0-beta.15.1 | |
| flarum-subscriptions | v0.1.0-beta.15 | |
| flarum-suspend | v0.1.0-beta.15 | |
| flarum-tags | v0.1.0-beta.15 | |
| fof-polls | 0.3.2 | |
| fof-user-directory | 0.5.0 | |
| fof-oauth | 0.2.1 | |
| flarum-likes | v0.1.0-beta.15 | |
| flarum-nicknames | v0.1.0-beta.15 | |
| fof-nightmode | 0.7.1 | |
+----------------------+------------------+--------+
Base URL: https://workinghours.timopartl.com
Debug mode: off
```

Reproduce the issue by enabling Dark Mode, turning on Colored header with #5572F7 in admin settings, and checking the main page. Trace the styles responsible for text and icon colors on colored backgrounds. Done means those elements remain bright and readable in dark mode without changing the configured background color.

Written by the indexing model from the issue text.