Fix theme selector resetting the buffer size (#25425)

Closes https://github.com/zed-industries/zed/issues/25413

As the issue points out well, themes do not need to alter any in-memory
state on load: that is done via settings file load.
Originally, it was introduced in
https://github.com/zed-industries/zed/pull/4064 and
https://github.com/zed-industries/zed/pull/24857 had restored that
behavior, which seems wrong to do.

Apart from removing that part, removes unnecessary methods and
emphasizes that in-memory state is the Buffer/UI size — no need to add
`Adjusted` there as the settings file presence is already enough.

Release Notes:

- Fixed theme selector resetting the buffer size

crates/assistant2/src/message_editor.rs

@@ -13,7 +13,7 @@ use rope::Point;
 use settings::Settings;
 use std::time::Duration;
 use text::Bias;
-use theme::{get_ui_font_size, ThemeSettings};
+use theme::ThemeSettings;
 use ui::{
     prelude::*, ButtonLike, KeyBinding, PopoverMenu, PopoverMenuHandle, Switch, TintColor, Tooltip,
 };
@@ -369,7 +369,7 @@ impl Render for MessageEditor {
                             .anchor(gpui::Corner::BottomLeft)
                             .offset(gpui::Point {
                                 x: px(0.0),
-                                y: (-get_ui_font_size(cx) * 2) - px(4.0),
+                                y: (-ThemeSettings::get_global(cx).ui_font_size(cx) * 2) - px(4.0),
                             })
                             .with_handle(self.inline_context_picker_menu_handle.clone()),
                     )