gui: Make warning titles more readable in Dark Theme

[norgeous]

before:

after:

[AudriusButkevicius]

@st-review merge

[calmh]

@st-review merge it

[st-review]

👌 Merged as 8709fec. Thanks, @norgeous!

[st-review]

@calmh: Merge failed:

$ git fetch -f origin refs/pull/3253/head:pr-3253
$ git fetch -f origin master:orig/master
From github.com:syncthing/syncthing
   48245ef..8709fec  master     -> orig/master
$ git reset --hard
HEAD is now at 8709fec gui: Make warning titles more readable in Dark Theme
$ git checkout master
Already on 'master'
Your branch is up-to-date with 'origin/master'.
$ git reset --hard orig/master
HEAD is now at 8709fec gui: Make warning titles more readable in Dark Theme
$ git clean -fxd
Skipping repository syncthing/syncthing
Skipping repository syncthing/syncthing-android
Skipping repository syncthing/docs
Skipping repository syncthing/discosrv
Skipping repository syncthing/syncthing-gtk
Skipping repository syncthing/syncthing-inotify
$ git merge --squash --no-commit pr-3253
Automatic merge went well; stopped before committing as requested
Squash commit -- not updating HEAD
$ git commit -F -
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean

[calmh]

⚡ :D