feat: extract more CSS variables

[jiewenhuang]

为了更好的适配主题样式，新增以下CSS变量：

--halo-search-widget-font-size-search-form-input:;                     /*输入框字体大小*/
--halo-search-widget-line-height-search-form-input:;                   /*输入框行高*/
--halo-search-widget-font-size-search-form-result-item-title:;         /*搜索结果项标题字体大小*/
--halo-search-widget-line-height-search-form-result-item-title:;       /*搜索结果项标题行高*/
--halo-search-widget-font-size-search-form-result-item-content:;       /*搜索结果项内容字体大小*/
--halo-search-widget-line-height-search-form-result-item-content:;     /*搜索结果项内容行高*/
--halo-search-widget-font-size-search-form-empty:;                     /*搜索结果为空时的字体大小*/
--halo-search-widget-line-height-search-form-empty:;                   /*搜索结果为空时的行高*/
--halo-search-widget-font-size-search-form-commands-item:;             /*搜索框底部快捷键提示字体大小*/
--halo-search-widget-line-height-search-commands-item:;                /*搜索框底部快捷键提示行高*/
--halo-search-widget-font-size-search-form-commands-item-kbd:;         /*搜索框底部快捷键提示图标大小*/ 
--halo-search-widget-min-width-search-commands-item-kbd:;              /*搜索框底部快捷键提示图标 min-wight*/
--halo-search-widget-font-family-search-form:;                         /*搜索框字体*/

新增 CSS 变量以更好地适配主题样式

[JohnNiang]

/ping @halo-dev/sig-halo-console

[ruibaby]

/lgtm

[JohnNiang]

/approve

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [JohnNiang]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment