Custom theme is not not applied

[blelem]

ALL software version info

Version 0.12.6. Problem can also be reproed using latest code available in the repo.

Description of expected behavior and the observed behavior

When using a css theme that is not located the BUNDLE_DIR, the BasicTemplate ignores the provided value, and uses the base_css instead.

Complete, minimal, self-contained example code that reproduces the issue

This is closely following the example from https://panel.holoviz.org/user_guide/Templates.html:

import param
import pathlib
from panel.template import BootstrapTemplate
from panel.template.theme import DefaultTheme, Theme

class BootstrapAcfTheme(DefaultTheme):
     # To repro the issue, the css provided here should not be in the BUNDLE_DIR
    css = param.Filename(default=pathlib.Path(__file__).parent / 'bootstrap-acf.css')
    _template = BootstrapTemplate

#Providing my own theme to the BootstrapTemplate
bootstrap_theme = BootstrapTemplate(title='Autocorrelation function', theme=BootstrapAcfTheme)
bootstrap_theme.show()

Upon rendering the page, the {{ raw_css }} tag from the template is replaced by the base_css instead of the content of bootstrap-acf.css. The served web page contains the following snippet, which comes from \panel\template\theme\default.css

...
<style type="text/css">
#header {
  background-color: #00aa41;
}
</style>

Code

This issue seems to be due to a cut and paste error. Line 620 uses theme.base_css, I am quite certain that it should use theme.css instead. Line 620 is an exact copy of line 613.

panel/panel/template/base.py

Lines 615 to 621 in 007475d

	if theme.css:
	basename = os.path.basename(theme.css)
	if (BUNDLE_DIR / name / basename).is_file():
	css_files['theme'] = dist_path + f'bundled/{name}/{basename}'
	else:
	with open(theme.base_css, encoding='utf-8') as f:
	raw_css.append(f.read())

I'll provide a PR that corrects this.